Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2018	Mar. 01, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	RCUS
Entity Registrant Name	Arcus Biosciences, Inc.
Entity Central Index Key	0001724521
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		44,534,594
Entity Public Float			$ 446,627,012

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 71,064	$ 98,426
Short-term investments	185,480	77,277
Prepaid expenses and other current assets	2,321	1,141
Amounts owed by a related party	83	25
Total current assets	258,948	176,869
Long-term investments	3,181
Property and equipment, net	11,107	11,230
Equity investment in related party	1,202	682
Restricted cash	203	203
Other long-term assets	284	1,502
Total assets	274,925	190,486
Current liabilities:
Accounts payable	3,102	3,820
Accrued liabilities	6,023	3,137
Deferred revenue, current	6,250	5,000
Other current liabilities	1,560	769
Total current liabilities	16,935	12,726
Deferred revenue, noncurrent	16,984	18,587
Deferred rent	4,272	4,740
Other long-term liabilities	1,792	565
Total liabilities	39,983	36,618
Commitments (Note 12)
Convertible preferred stock, $0.0001 par value, no shares and 120,958,867 shares authorized as of December 31, 2018 and 2017, respectively; no shares and 30,459,574 shares issued and outstanding as of December 31, 2018 and 2017, respectively	0	226,196
Stockholders’ equity (deficit):
Preferred stock, $0.0001 par value, 10,000,000 shares authorized as of December 31, 2018; no shares issued and outstanding	0
Common stock, $0.0001 par value, 400,000,000 and 153,993,227 shares authorized as of December 31, 2018 and 2017, respectively; 44,537,946 and 4,090,898 shares issued and outstanding as of December 31, 2018 and 2017, respectively	4
Additional paid-in capital	357,873	948
Accumulated deficit	(122,828)	(73,234)
Accumulated other comprehensive loss	(107)	(42)
Total stockholders’ equity (deficit)	234,942	(72,328)
Total liabilities, convertible preferred stock and stockholders’ equity	$ 274,925	$ 190,486

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Convertible preferred stock, par value	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	0	120,958,867
Convertible preferred stock, shares issued	0	30,459,574
Convertible preferred stock, shares outstanding	0	30,459,574
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	400,000,000	153,993,227
Common stock, shares issued	44,537,946	4,090,898
Common stock, shares outstanding	44,537,946	4,090,898

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Collaboration and license revenue	$ 8,353	$ 1,413
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operation expenses:
Research and development	$ 49,646	$ 47,218	$ 14,247
General and administrative	13,566	7,636	3,935
Total operating expenses	63,212	54,854	18,182
Loss from operations	(54,859)	(53,441)	(18,182)
Non-operating income (expense):
Interest and other income (expense), net	4,922	775	212
Gain on deemed sale from equity method investee	1,229	0	0
Share of loss from equity method investee	(886)	(416)	0
Total non-operating income, net	5,265	359	212
Net loss	(49,594)	(53,082)	(17,970)
Other comprehensive loss	(65)	(16)	(26)
Comprehensive loss	$ (49,659)	$ (53,098)	$ (17,996)
Net loss per share, basic and diluted	$ (1.43)	$ (29.03)	$ (20.80)
Weighted-average number of shares used to compute basic and diluted net loss per share	34,618,237	1,828,262	863,983

Consolidated Statements of Convertible Preferred Stock and Stockholders' Deficit - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Series B Convertible Preferred Stock	Series C Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss
Balance at Dec. 31, 2015	$ (2,171)					$ 11	$ (2,182)
Balance, shares at Dec. 31, 2015		12,556,791
Balance at Dec. 31, 2015		$ 49,637
Balance, shares at Dec. 31, 2015					2,798,146
Issuance of convertible preferred stock, net of issuance costs			$ 69,817
Issuance of convertible preferred stock, net of issuance costs, shares			8,750,852
Vesting of early exercised stock options	63					63
Vesting of early exercised stock options, shares					137,050
Stock-based compensation	90					90
Other comprehensive loss	(26)							$ (26)
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares	$ 20					20
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares, shares	637,107				47,576
Net loss	$ (17,970)					0	(17,970)
Balance at Dec. 31, 2016	(19,994)					184	(20,152)	(26)
Balance, shares at Dec. 31, 2016		21,307,643
Balance at Dec. 31, 2016		$ 119,454
Balance, shares at Dec. 31, 2016					2,982,772
Issuance of convertible preferred stock, net of issuance costs				$ 106,742
Issuance of convertible preferred stock, net of issuance costs, shares				9,151,931
Vesting of early exercised stock options and restricted stock	235					235
Vesting of early exercised stock options and restricted stock, shares					269,752
Stock-based compensation	495					495
Other comprehensive loss	(16)							(16)
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares	$ 34					34
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares, shares	643,024				25,605
Net loss	$ (53,082)						(53,082)
Balance at Dec. 31, 2017	$ (72,328)					948	(73,234)	(42)
Balance, shares at Dec. 31, 2017	30,459,574	30,459,574
Balance at Dec. 31, 2017	$ 226,196	$ 226,196
Balance, shares at Dec. 31, 2017	4,090,898				3,278,129
Conversion of preferred stock to common stock	$ 226,198				$ 3	226,195
Conversion of preferred stock to common stock, shares		(30,459,574)
Conversion of preferred stock to common stock		$ (226,196)
Conversion of preferred stock to common stock, shares					30,459,574
Issuance of common stock upon IPO, net of issuance costs	124,735				$ 1	124,734
Issuance of common stock upon IPO, net of issuance costs, shares					9,200,000
Vesting of early exercised stock options and restricted stock	1,276					1,276
Vesting of early exercised stock options and restricted stock, shares					528,374
Issuance of common stock under Employee Stock Purchase Plan	751					751
Issuance of common stock under Employee Stock Purchase Plan, shares					77,397
Stock-based compensation	3,874					3,874
Other comprehensive loss	(65)							(65)
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares	$ 95					95
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares, shares	720,756				67,349
Net loss	$ (49,594)						(49,594)
Balance at Dec. 31, 2018	$ 234,942				$ 4	$ 357,873	$ (122,828)	$ (107)
Balance, shares at Dec. 31, 2018	0	0
Balance at Dec. 31, 2018	$ 0
Balance, shares at Dec. 31, 2018	44,537,946				43,610,823

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flow from operating activities
Net loss	$ (49,594)	$ (53,082)	$ (17,970)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	3,874	495	90
Depreciation and amortization	3,664	2,612	1,314
Share of loss from equity method investee	886	416	0
Gain on deemed sale from equity method investee	(1,229)	0	0
(Accretion of discounts) amortization of premiums on investments	(1,752)	0	0
Other non-operating income	(177)	(98)	0
Changes in operating assets and liabilities
Amounts owed by a related party	(58)	380	0
Prepaid expenses and other current assets	(1,180)	(751)	(602)
Other long-term assets	(80)	(6)	(200)
Accounts payable	(69)	(267)	3,569
Accrued liabilities	3,497	1,582	808
Other current liabilities	43	(136)	647
Deferred revenue	(353)	23,587	0
Deferred rent	(468)	209	(600)
Net cash used in operating activities	(42,996)	(25,059)	(12,944)
Cash flow from investing activities
Purchases of short-term and long-term investments	(261,552)	(96,830)	(33,762)
Proceeds from maturities of short-term and long-term investments	151,855	53,273	0
Purchases of property and equipment	(3,743)	(5,514)	(4,099)
Investment in related party	0	0	(1,000)
Net cash used in investing activities	(113,440)	(49,071)	(38,861)
Cash flow from financing activities
Proceeds from initial public offering, net of issuance costs	125,111	0	0
Proceeds from issuance of preferred stock, net of issuance costs	0	106,877	69,817
Proceeds from issuance of common stock upon exercise of stock options, net of repurchases	4,098	892	283
Deferred initial public offering costs	0	(373)	0
Payment of preferred stock issuance costs	(135)	0	0
Net cash provided by financing activities	129,074	107,396	70,100
Net decrease in cash and cash equivalents	(27,362)	33,266	18,295
Cash and cash equivalents at beginning of period	98,426	65,160	46,865
Cash and cash equivalents at end of period	71,064	98,426	65,160
Non-cash investing and financing activities:
Unpaid financing cost included in accounts payable and accrued liabilities	0	1,058	0
Unpaid portion of property and equipment purchases included in accounts payable and accrued liabilities	136	338	618
Vesting of early exercised stock options and restricted stock	$ 1,276	$ 235	$ 63

Organization

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Organization

Note 1. Organization

Description of Business

Arcus Biosciences, Inc. (Company) is a clinical-stage biopharmaceutical company focused on creating innovative cancer immunotherapies by leveraging underexploited biological opportunities. Specifically, the Company targets well-characterized biological pathways with significant scientific data supporting their importance in regulating the immune response against cancer and for which either there are no molecules in development or those that exist have suboptimal profiles. To exploit these pathways, the Company has built a robust and highly efficient discovery capability to create and optimize highly differentiated small-molecule immuno-oncology product candidates. Since its inception in 2015, the Company has built a broad portfolio of small molecule and antibody product candidates that it plans to develop together as intra-portfolio combinations.

Initial Public Offering

On March 21, 2018, the Company completed its initial public offering (IPO) pursuant to which the Company issued 9,200,000 shares of common stock, including the exercise of the underwriters’ overallotment option to purchase 1,200,000 shares of common stock, at an offering price at $15.00 per share. The Company received aggregate net proceeds of approximately $124.7 million after deducting underwriting discounts and other offering related costs. In addition, in connection with the completion of the Company’s IPO, all outstanding shares of convertible preferred stock were converted into 30,459,574 shares of common stock, and the Company amended and restated its certificate of incorporation and bylaws, which, among other things, changed the authorized capital stock to 400,000,000 shares of common stock and 10,000,000 shares of preferred stock, each with a par value of $0.0001 per share.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

Note 2. Summary of Significant Accounting Policies

Basis of Presentation

The consolidated financial statements and accompanying notes have been prepared in accordance with generally accepted accounting principles in the United States of America (U.S. GAAP) and include all adjustments necessary for the fair presentation of the Company’s financial position for the periods presented.

Principles of Consolidation

During 2017, the Company established a wholly-owned subsidiary in Australia. The consolidated financial statements include the Company’s accounts and those of its wholly-owned subsidiary. All intercompany accounts, transactions and balances have been eliminated.

Reverse Stock Split

On March 9, 2018, the Company effected a reverse split of all shares of its common and preferred stock at a ratio of 1-for-3.96 (the Reverse Split). The par values and the authorized shares of the common and preferred stock were not adjusted as a result of the Reverse Split. All references to shares of common stock outstanding, average number of shares outstanding and per share amounts in these consolidated financial statements and notes to the consolidated financial statements have been adjusted within the consolidated financial statements, on a retroactive basis, to reflect the Reverse Split.

Use of Estimates

The preparation of the Company’s consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, as well as related disclosure of contingent assets and liabilities. Estimates were used to determine the fair value of common stock prior to the IPO and are used to determine stock-based awards and other issuances, accruals for research and development costs, useful lives of long-lived assets, and uncertain tax positions. Actual results could differ materially from the Company’s estimates.

Risk and Uncertainties

The Company’s future results of operations involve a number of risks and uncertainties. Factors that could affect the Company’s future operating results and cause actual results to vary materially from expectations include, but are not limited to, uncertainty of results of clinical trials and reaching milestones, uncertainty of regulatory approval of the Company’s potential drug candidates, uncertainty of market acceptance of the Company’s product candidates, competition from substitute products and larger companies, securing and protecting proprietary technology, strategic relationships and dependence on key individuals and sole source suppliers.

The Company’s product candidates require approvals from the U.S. Food and Drug Administration (FDA) and comparable foreign regulatory agencies prior to commercial sales in their respective jurisdictions. There can be no assurance that any product candidates will receive the necessary approvals. If the Company does not obtain approval and does not successfully commercialize any of its product candidates, it would have a materially adverse impact on the Company.

Segments

The Company operates and manages its business as one reportable and operating segment, which is the business of developing and commercializing immunotherapies. The Company’s chief executive officer, who is the chief operating decision maker, reviews financial information on an aggregate basis for purposes of allocating and evaluating financial performance. All long-lived assets are maintained in the United States of America.

Cash Equivalents, Short-Term and Long-Term Investments

Cash equivalents include marketable securities having an original maturity of three months or less at the time of purchase. Short-term investments have maturities of greater than three months at the time of purchase. Long-term investments have maturities greater than 12 months at the time of purchase. Collectively, cash equivalents, short-term and long-term investments are considered available-for-sale and are recorded at fair value. Unrealized gains and losses are recorded in accumulated other comprehensive loss. Realized gains and losses are included in interest and other income, net in the consolidated statements of operations and comprehensive loss. The basis on which the cost of a security sold or amount reclassified out of accumulated other comprehensive income into earnings is determined using the specific identification method.

Restricted Cash

Restricted cash at December 31, 2018 and 2017 comprises cash balances primarily held as security in connection with the Company’s facility lease agreement and is included in long-term assets in the consolidated balance sheets.

Receivable From a Related Party

Receivable from a related party is recorded net of any allowances. As of December 31, 2018 and 2017, the outstanding amount is due from PACT Pharma, Inc. (PACT Pharma) for expenses the Company paid for on its behalf. The Company is exposed to credit risk in the event of a default by PACT Pharma. To date, the Company has not experienced any losses related to these receivables (see Note 5) and has not recorded any allowances.

Fair Value Measurements

Fair value accounting is applied for all financial assets and liabilities, including short-term and long-term investments, and non-financial assets and liabilities that are recognized or disclosed at fair value in the consolidated financial statements on a recurring basis (at least annually). The carrying amount of the Company’s financial instruments, including receivable from a related party, accounts payable and accrued expenses and other current liabilities approximate fair value due to their short-term maturities.

Concentration of Credit Risk

Cash equivalents, short-term and long-term investments are financial instruments that potentially subject the Company to concentrations of credit risk. The Company invests in money market funds, treasury bills and notes, government bonds, commercial paper and corporate notes. The Company limits its credit risk associated with cash equivalents, short-term and long-term investments by placing them with banks and institutions it believes are highly credit worthy and in highly rated investments.

Property and Equipment

Property and equipment are stated at cost and depreciated using the straight-line method over the estimated useful lives of the assets, ranging from one to five years. Leasehold improvements are amortized over the shorter of their estimated useful lives or the related lease term. Upon retirement or sale, the cost and related accumulated depreciation are removed from the consolidated balance sheet and the resulting gain or loss is reflected in the consolidated statement of operations and comprehensive loss.

Impairment of Long-Lived Assets

The Company reviews long-lived assets, including property and equipment, for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. An impairment charge would be recorded when estimated undiscounted future cash flows expected to result from the use of the asset and its eventual disposition are less than its carrying amount. Impairment, if any, is assessed using discounted cash flows or other appropriate measures of fair value. The Company did not recognize any impairment charges for the years ended December 31, 2018 and 2017.

Deferred Offering Costs

Deferred offering costs associated with the Company’s IPO, consisting of legal, accounting, filing and other fees directly related to the IPO, were capitalized. The deferred offering costs, which totaled $3.6 million, were reclassified to additional paid-in capital upon the effectiveness of the IPO in March 2018. As of December 31, 2017, $1.3 million of deferred offering costs were capitalized and included in other long-term assets in the consolidated balance sheet.

Revenue Recognition

The Company generates revenue from its option and license agreement for the development and commercialization of its product candidates. Option and license agreements may include non-refundable upfront research and development fees, option fees to obtain development and commercialization licenses for the Company’s products, milestone payments based on achievement of defined development, regulatory and sales targets, and royalties on sales of commercialized products. To date, the Company has not recognized revenue from sales of its product candidates.

The Company recognizes revenue when all four of the following criteria have been met: (i) collectability is reasonably assured; (ii) delivery has occurred or services have been rendered; (iii) persuasive evidence of an arrangement exists; and (iv) the fee is fixed or determinable. Revenue under option and license arrangements is recognized based on evaluation of the performance obligations of the contract. Collectability is assessed based on evaluation of payment criteria as stated in the contract as well as the creditworthiness of the customer. Determination of whether delivery has occurred, or services rendered are based on management’s evaluation of the performance obligations as stated in the contract and progress made against those obligations. Evidence of an arrangement is deemed to exist upon execution of the contract. Fees are considered fixed and determinable when the amount payable to the Company is no longer subject to any acceptance, refund rights or other contingencies that would alter the fixed nature of the fees charged for the deliverables.

Option and license agreements may contain multiple elements as evaluated under Accounting Standards Codification (ASC) 605-25, Revenue Recognition- Multiple-Element Arrangements, including agreements to provide research and development services, participation in development and/or steering committees, manufacturing services, sharing of know-how and other information, and grants of licenses to develop and commercialize product candidates. Each deliverable under the agreement is evaluated to determine whether it qualifies as a separate unit of accounting based on whether the deliverable has standalone value to the customer. The arrangement’s consideration that is fixed or determinable is then allocated to each separate unit of accounting based on the following hierarchy: (i) vendor-specific objective evidence of the fair value of the deliverable, if it exists; (ii) third-party evidence of selling price, if vendor-specific objective evidence is not available; or (iii) the best estimate of selling price if neither vendor-specific objective evidence or third-party evidence is available.

A delivered item or items that do not qualify as a separate unit of accounting within the arrangement are combined with the other applicable undelivered items within the arrangement. The allocation of arrangement consideration and the recognition of revenue is then determined for those combined deliverables as a single unit of accounting. For a combined unit of accounting, non-refundable upfront fees are recognized as performance obligations related to the final deliverable are completed. In the case of research and development services, performance would generally be expected to be ratable over the estimated performance period unless the Company determines there is a discernible pattern of performance other than straight-line, in which case the Company uses a proportionate performance method to recognize the revenue over the estimated performance period. Amounts received in advance of performance are recorded as deferred revenue. If any of the initial deliverables are determined to have standalone value separate from the research and development services, then the allocated consideration is recorded as revenue when those items are delivered.

Options to acquire development and commercialization licenses of the Company’s products are evaluated to determine if they are substantive. Fees for substantive options are recognized as revenue when an option is exercised by the collaboration partner and the Company has completed the deliverables that are associated with exercise of such option.

Option and license agreements may also contain milestone payments that become due upon the achievement of certain milestones. The Company applies ASC 605-28, Revenue Recognition—Milestone Method. Under the milestone method, payments that are contingent upon achievement of a substantive milestone are recognized in the period in which the milestone is achieved. Milestones are defined as an event that can only be achieved based on the Company’s performance and there is substantive uncertainty about whether the event will be achieved at the inception of the arrangement. Events that are contingent only on the passage of time or only on counterparty performance are not considered milestones subject to this guidance. Further, for the milestone to be considered substantive, the amounts received must relate solely to prior performance, be reasonable relative to all of the deliverables, and the consideration must be commensurate with the Company’s performance to achieve the milestone. Non-substantive milestone payments are recognized as revenue over the estimated period of any remaining performance obligations.

Research and Development Expenses

Research and development costs are expensed as incurred. Research and development expenses consist primarily of personnel costs for the Company’s research and product development employees. Also included are non-personnel costs such as professional fees payable to third parties for preclinical and clinical studies and research services, laboratory supplies and equipment maintenance, product licenses, and other consulting costs.

The Company estimates preclinical and clinical study and research expenses based on the services performed, pursuant to contracts with research institutions that conduct and manage preclinical and clinical studies and research services on its behalf. The Company estimates these expenses based on discussions with internal management personnel and external service providers as to the progress or stage of completion of services and the contracted fees to be paid for such services. If the actual timing of the performance of services or the level of effort varies from the original estimates, the Company will adjust the accrual accordingly. Payments associated with licensing agreements to acquire exclusive licenses to develop, use, manufacture and commercialize products that have not reached technological feasibility and do not have alternative future use are expensed as incurred. Payments made to third parties under these arrangements in advance of the performance of the related services by the third parties are recorded as prepaid expenses until the services are rendered.

Leases and Rent Expense

The Company records rent expense on a straight-line basis over the life of the lease. In cases where there is a free rent period or future fixed rent escalations, the Company records a deferred rent liability. Additionally, the receipt of any lease incentives is recorded as a deferred rent liability which is amortized over the lease term as a reduction of rent expense. Any lease incentives that are due from the landlord but have not been collected are recorded as a receivable in Prepaid expenses and other current assets. Building improvements made with the lease incentives or tenant allowances are capitalized as leasehold improvements and included in property and equipment in the consolidated balance sheets.

Stock-Based Compensation

The Company accounts for stock-based compensation arrangements with employees in accordance with ASC 718, Stock Compensation. Stock-based awards granted include stock options with time-based vesting. ASC 718 requires the recognition of compensation expense, using a fair value-based method, for costs related to all stock-based payments. The Company’s determination of the fair value of stock options with time-based vesting on the date of grant utilizes the Black-Scholes option-pricing model, and is impacted by its common stock price as well as other variables including, but not limited to, expected term that options will remain outstanding, expected common stock price volatility over the term of the option awards, risk-free interest rates and expected dividends.

The fair value of a stock-based award is recognized over the period during which an optionee is required to provide services in exchange for the option award, known as the requisite service period (usually the vesting period) on a straight-line basis. Stock-based compensation expense is recognized based on the fair value determined on the date of grant and is reduced for forfeitures as they occur.

Equity instruments issued to non-employees are accounted for in accordance with ASC 505-50, Equity Based Payments to Non-Employees, and are recorded at their fair value on the measurement date and are subject to periodic adjustments as the underlying equity instruments vest. The fair value of options granted to consultants is expensed when vested. Non-employee stock-based compensation expense was not material for all periods presented.

Estimating the fair value of equity-settled awards as of the grant date using valuation models, such as the Black-Scholes option pricing model, is affected by assumptions regarding a number of complex variables. Changes in the assumptions can materially affect the fair value and ultimately how much stock-based compensation expense is recognized. These inputs are subjective and generally require significant analysis and judgment to develop.

Convertible Preferred Stock

The Company records all shares of convertible preferred stock at their respective fair values less issuance costs on the dates of issuance. The convertible preferred stock is recorded outside of stockholders’ equity (deficit) because, in the event of certain deemed liquidation events considered not solely within the Company’s control, such as a merger, acquisition and sale of all or substantially all of the Company’s assets, the convertible preferred stock will become redeemable at the option of the holders. In the event of a change of control of the Company, proceeds received from the sale of such shares will be distributed in accordance with the liquidation preferences set forth in the Company’s Amended and Restated Certificate of Incorporation unless the holders of convertible preferred stock have converted their shares of convertible preferred stock into shares of common stock. The Company has determined not to adjust the carrying values of the convertible preferred stock to the liquidation preferences of such shares because of the uncertainty of whether or when such an event would occur. All of outstanding convertible preferred stock converted into common stock in March 2018 upon the effectiveness of the IPO.

Income Taxes

The Company provides for income taxes under the asset and liability method. Current income tax expense or benefit represents the amount of income taxes expected to be payable or refundable for the current year. Deferred income tax assets and liabilities are determined based on differences between the financial statement reporting and tax bases of assets and liabilities and net operating loss and credit carryforwards, and are measured using the enacted tax rates and laws that will be in effect when such items are expected to reverse. Deferred income tax assets are reduced, as necessary, by a valuation allowance when management determines it is more likely than not that some or all of the tax benefits will not be realized.

The Company accounts for uncertain tax positions in accordance with ASC 740-10, Accounting for Uncertainty in Income Taxes. The Company assesses all material positions taken in any income tax return, including all significant uncertain positions, in all tax years that are still subject to assessment or challenge by relevant taxing authorities. Assessing an uncertain tax position begins with the initial determination of the position’s sustainability and is measured at the largest amount of benefit that is greater than fifty percent likely of being realized upon ultimate settlement. As of each balance sheet date, unresolved uncertain tax positions must be reassessed, and the Company will determine whether (i) the factors underlying the sustainability assertion have changed and (ii) the amount of the recognized tax benefit is still appropriate. The recognition and measurement of tax benefits requires significant judgment. Judgments concerning the recognition and measurement of a tax benefit might change as new information becomes available.

The Company includes any penalties and interest expense related to income taxes as a component of other expense and interest expense, net, as necessary.

Comprehensive Loss

Comprehensive loss is defined as a change in equity of a business enterprise during a period, resulting from transactions from non-owner sources. The Company had unrealized loss from its available-for-sale securities during the years ended December 31, 2018, 2017 and 2016, which meets the criteria as other comprehensive loss and, therefore, the Company has reported comprehensive loss and net loss.

Net Loss per Share

Basic net loss per share is calculated by dividing the net loss by the weighted-average number of common shares outstanding during the period, without consideration for potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss by the weighted-average number of common shares and potentially dilutive securities outstanding for the period. For purposes of the diluted net loss per share calculation, convertible preferred stock, and common stock options are considered to be potentially dilutive securities. Because the Company reported a net loss for the years ended December 31, 2018, 2017 and 2016, and the inclusion of the potentially dilutive securities would be antidilutive, diluted net loss per share is the same as basic net loss per share for all periods.

Recent Accounting Pronouncements

From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (FASB), or other standard setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the impact of recently issued standards that are not yet effective will not have a material impact on the Company’s consolidated financial statements upon adoption. Under the Jumpstart Our Business Startups Act of 2012, as amended (the JOBS Act), the Company meets the definition of an emerging growth company, and has elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act. The Company did not adopt any new accounting standards during 2018.

Recently Issued Accounting Standards or Updates Not Yet Effective

In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (ASU 2014-09). The FASB issued numerous updates that provide clarification on a number of specific issues as well as requiring additional disclosures. Collectively, the new revenue standards became effective for public business entities for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, ASU 2014-09 will be effective for the Company for the year ended December 31, 2019, and all interim periods thereafter. Early adoption is permitted. ASU 2014-09 also permits two methods of adoption: retrospectively to each prior reporting period presented (full retrospective method), or retrospectively with the cumulative effect of initially applying the guidance recognized at the date of initial application (the modified retrospective method).

The core principle of ASU 2014-09 is that an entity should recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. ASU 2014-09 defines a five-step process to achieve this core principle and, in doing so, it is possible more judgment and estimates may be required within the revenue recognition process than required under existing U.S. generally accepted accounting pronouncements. The Company has substantially completed its evaluation related to the adoption of ASU 2014-09, applying the five-step model of the new standard to the option and license agreement with Taiho Pharmaceutical Co., Ltd. (Taiho), the only agreement which will be impacted by the adoption of the new standard, specifically as it pertains to the non-refundable, non-creditable upfront cash payments to the Company totaling $35.0 million, the option exercise payment of $3.0 million and all other future contingent payments the Company may become entitled to. The Company will finalize its accounting assessment and quantitative impact of the adoption during the first quarter of fiscal year 2019, using the modified retrospective method, which will reflect the cumulative effect of the adoption retrospectively as of January 1, 2019, the initial date of adoption.

In January 2016, the FASB issued ASU No. 2016-01 (Subtopic 825-10), Financial Instruments (ASU 2016-01). ASU 2016-01 requires management to measure marketable investments at fair value with changes in fair value recognized in net income or loss. ASU 2016-01 will impact the disclosure and presentation of financial assets and liabilities. ASU 2016-01 was effective for public business entities for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, ASU 2016-01 will be effective for the Company as of December 31, 2019, and all interim periods within. Early adoption is not permitted. The adoption of this standard is not expected to have a material impact on the Company’s consolidated financial statements.

In February 2016, the FASB issued ASU No. 2016-02 (Topic 842), Leases (ASU 2016-02). ASU 2016-02 requires an entity to recognize assets and liabilities arising from a lease for both financing and operating leases. The ASU will also require new qualitative and quantitative disclosures to help investors and other financial statement users better understand the amount, timing, and uncertainty of cash flows arising from leases. For public entities, ASU 2016-02 is effective for fiscal years beginning after December 15, 2018. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, ASU 2016-02 is effective for the Company for the year ended December 31, 2020, and all interim periods within. Early adoption is permitted. The Company has not yet determined the potential effects of this ASU on its consolidated financial statements.

In November 2016, the FASB issued ASU No. 2016-18 (Topic 230), Restricted Cash, Statement of Cash Flows (ASU 2016-18). ASU 2016-18 requires that a statement of cash flows explain the change during the period in the total of cash and, cash equivalents, and amounts generally described as restricted cash or restricted cash equivalents. Therefore, amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the consolidated statement of cash flows. ASU 2016-18 was effective for public business entities for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, ASU 2016-18 will be effective for the Company for the year ended December 31, 2019, and all interim periods thereafter. Early adoption is permitted. The amendments in this ASU should be applied using a retrospective transition method to each period presented. The adoption of this standard is not expected to have a material impact on the Company’s consolidated financial statements.

In June 2018, the FASB issued ASU No. 2018-07 (Topic 718), Compensation – Stock Compensation (ASU 2018-07). ASU 2018-07 requires an entity to expand the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from nonemployees. For public entities, ASU 2018-07 is effective for fiscal years beginning after December 15, 2018. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, ASU 2018-07 is effective for the Company for the year ended December 31, 2020, and all interim periods within. Early adoption is permitted. The Company has not yet determined the potential effects of this ASU on its consolidated financial statements.

In August 2018, the FASB issued ASU No.2018-13 (Topic 820), Fair Value Measurement. ASU 2018-13 modifies the disclosure requirements on fair value measurement in Topic 820. For public entities, ASU 2018-013 is effective for fiscal years beginning after December 15, 2019. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, ASU 2018-13 is effective for the Company for the year ended December 31, 2020, and all interim periods within. Early adoption is permitted. The adoption of this standard is not expected to have a material impact on the Company’s consolidated financial statements.

Fair Value Measurements

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements

Note 3. Fair Value Measurements

Financial assets and liabilities are recorded at fair value. The accounting guidance for fair value provides a framework for measuring fair value, clarifies the definition of fair value and expands disclosures regarding fair value measurements. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability (an exit price) in an orderly transaction between market participants at the reporting date. The accounting guidance establishes a three-tiered hierarchy, which prioritizes the inputs used in the valuation methodologies in measuring fair value as follows:

Level 1—Inputs are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date.

Level 2—Inputs (other than quoted market prices included in Level 1) are either directly or indirectly observable for the asset or liability through correlation with market data at the measurement date and for the duration of the instrument’s anticipated life.

Level 3—Inputs reflect management’s best estimate of what market participants would use in pricing the asset or liability at the measurement date. Consideration is given to the risk inherent in the valuation technique and the risk inherent in the inputs to the model.

Assets and liabilities measured at fair value are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. The Company’s assessment of the significance of a particular input to the fair value measurement in its entirety requires management to make judgments and consider factors specific to the asset or liability.

During the periods presented, the Company has not changed the manner in which it values assets and liabilities that are measured at fair value. The Company recognizes transfers between levels of the fair value hierarchy as of the end of the reporting period. There were no transfers within the hierarchy as of December 31, 2018 and 2017. The following tables set forth the Company’s financial instruments that were measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands):

	Fair Value Measurements at December 31, 2018
	Total		Level 1		Level 2		Level 3
Assets
Money market funds	$	45,017	$	45,017	$	—	$	—
U.S. government agency obligations		103,940		—		103,940		—
Corporate securities and commercial paper		110,768		—		110,768		—
	$	259,725	$	45,017	$	214,708	$	—

	Fair Value Measurements at December 31, 2017
	Total		Level 1		Level 2		Level 3
Assets
Money market funds	$	66,478	$	66,478	$	—	$	—
U.S. government agency obligations		57,153		—		57,153		—
Corporate securities and commercial paper		52,072		—		52,072		—
	$	175,703	$	66,478	$	109,225	$	—

Classified as (with contractual maturities):

	December 31, 2018		December 31, 2017
Cash and cash equivalents	$	71,064	$	98,426
Short-term investments (due within one year)		185,480		77,277
Long-term investments (due between one and two years)		3,181		—
	$	259,725	$	175,703

The investments are classified as available-for-sale marketable securities. At December 31, 2018 and 2017, the balance in the Company’s accumulated other comprehensive loss comprised activity related to the Company’s available-for-sale marketable securities. There were no realized gains or losses recognized on the sale or maturity of available-for-sale marketable securities as of December 31, 2018 and 2017, and as a result, the Company did not reclassify any amounts out of accumulated other comprehensive loss for the periods then ended. The Company has a limited number of available-for-sale marketable securities in loss positions as of December 31, 2018, which the Company does not intend to sell and has concluded it will not be required to sell before recovery of the amortized cost for the investment at maturity (in thousands).

	Fair Value Measurements at December 31, 2018
	Amortized Cost		Unrealized Gain		Unrealized Loss			Fair Value
Money market funds	$	45,017	$	—	$	—		$	45,017
U.S. government agency obligations		103,957		—		(17	)		103,940
Corporate securities and commercial paper		110,859		—		(91	)		110,768
	$	259,833	$	—	$	(108	)	$	259,725

	Fair Value Measurements at December 31, 2017
	Amortized Cost		Unrealized Gain		Unrealized Loss			Fair Value
Money market funds	$	66,478	$	—	$	—		$	66,478
U.S. government agency obligations		57,183		—		(30	)		57,153
Corporate securities and commercial paper		52,084		—		(12	)		52,072
	$	175,745	$	—	$	(42	)	$	175,703

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Consolidated Balance Sheet Components

12 Months Ended

Dec. 31, 2018

Balance Sheet Related Disclosures [Abstract]

Consolidated Balance Sheet Components

Note 4. Consolidated Balance Sheet Components

Property and Equipment

Property and equipment, net consisted of the following (in thousands):

	As of December 31, 2018			As of December 31, 2017
Scientific equipment	$	6,628		$	5,053
Furniture and equipment		813			625
Capitalized software		146			131
Leasehold improvements		10,828			9,280
Construction in progress		335			119
Total		18,750			15,208
Less: Accumulated depreciation and amortization		(7,643	)		(3,978	)
Property and equipment, net	$	11,107		$	11,230

Depreciation and amortization expense was $3.7 million and $2.6 million for the years ended December 31, 2018 and 2017, respectively.

Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):

	As of December 31, 2018		As of December 31, 2017
Accrued personnel expenses	$	2,833	$	1,026
Accrued research and development expenses		2,816		1,193
Professional fees		211		706
Other		163		212
Total	$	6,023	$	3,137

Equity Investment

12 Months Ended

Dec. 31, 2018

Inventory Disclosure [Abstract]

Equity Investment

Note 5: Equity Investment

In September 2016, the Company purchased approximately 3.6 million shares of common stock of PACT Pharma, Inc. (PACT Pharma), a privately funded, early-stage biopharmaceutical company focused on adoptive cell therapy. The Company determined the fair value of such investment to be insignificant to the Company’s 2016 financial statements given the start-up nature of operations of PACT Pharma, and it was recorded at a nominal amount. In December 2016, the Company and PACT Pharma entered into a Master Services Agreement (the PACT Agreement) under which the Company provided PACT Pharma with general administrative support, including finance, human resources, legal, and other operational support. The Company also received certain warrants to purchase PACT Pharma common stock exercisable upon PACT Pharma’s achievement of certain valuation thresholds pursuant to the PACT Agreement. Also, in December 2016, the Company purchased 1.0 million shares of Series A preferred stock of PACT Pharma for $1.0 million. The Company determined PACT Pharma to be a variable interest entity, and that the Company has a variable interest in PACT. However, because the Company is not the primary beneficiary of PACT Pharma, it is not consolidating the results of operations of PACT Pharma in its consolidated financial statements.

The Company’s investment in PACT Pharma is accounted for as an equity method investment, and as a result the Company records its share of PACT Pharma’s operating results in share of loss from equity method investee in its consolidated statement of operations and comprehensive loss.

For the years ended December 31, 2018 and 2017, the Company recorded $0.9 million and $0.4 million, respectively relating to its share of PACT Pharma’s operating loss. As of December 31, 2018 and 2017, the Company had a $0.1 million and $25,000 receivable from PACT Pharma, respectively, for expenses the Company paid for on its behalf.

In May 2018, PACT Pharma closed its Series B convertible preferred stock financing. The Company did not participate in this financing and therefore its equity ownership percentage in PACT Pharma decreased. As a result of the dilution in its equity ownership percentage and an increase in PACT Pharma’s estimated fair value per share, the Company recorded a gain of $1.2 million in gain on deemed sale from equity method investee during the year ended December 31, 2018 and an increase in the fair value of the investment balance in the consolidated balance sheet as of December 31, 2018 by the same amount. The PACT Agreement also expired in accordance with its terms at the closing of PACT Pharma’s Series B convertible preferred stock financing.

The Company monitors the investment for events or circumstances indicative of potential other-than-temporary impairment and makes appropriate reductions in carrying values if it is determined that an impairment charge is required. As of December 31, 2018 and 2017, no impairment charge was recorded. For the years ended December 31, 2018 and 2017, the Company also determined the fair value of the warrants to be insignificant to the consolidated financial statements.

License and Collaboration Agreements

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

License and Collaboration Agreements

Note 6. License and Collaboration Agreements

Taiho Pharmaceutical Co., Ltd

In September 2017, the Company and Taiho entered into an option and license agreement (the Taiho Agreement) to collaborate on the potential development and commercialization of certain product candidates from the Company’s portfolio in Japan and certain other territories in Asia (excluding China) (the Taiho Territory). The Taiho Agreement provides Taiho with exclusive options, over a five-year period (the Option Period), to obtain an exclusive development and commercialization license to clinical stage product candidates from the Company’s programs (each, an Arcus Program).

In consideration for the exclusive options and other rights contained in the Taiho Agreement, Taiho will make non-refundable, non-creditable cash payments to the Company totaling $35.0 million, of which the Company received $25.0 million during 2017. An additional $5.0 million was received in October 2018 and the remaining $5.0 million is expected to be received in 2019.

In the event that the Company has not initiated IND enabling studies for at least five Arcus Programs prior to the expiration of the Option Period, Taiho may elect to extend the Option Period, up to a maximum of seven years for the Option Period, subject to an extension fee. If Taiho elects to exercise an option they will be obligated to make an exercise option payment for each option exercise of between $3.0 million to $15.0 million, dependent on the development stage of the applicable Arcus Program for which the option is exercised. In addition, the Taiho Agreement provides that the Company is eligible to receive additional clinical and regulatory milestones totaling up to $130.0 million per Arcus Program, and it will be eligible to receive contingent payments of up to $145.0 million per Arcus Program associated with the achievement of specified levels of Taiho net sales in the Taiho Territory.

In addition, the Company will receive royalties ranging from high single-digits to mid-teens on net sales of licensed products in the Taiho Territory. Royalties will be payable on a licensed product-by-licensed product and country-by-country basis during the period of time commencing on the first commercial sale of a licensed product in a country and ending upon the later of: (a) ten (10) years from the date of first commercial sale of such licensed product in such country; and (b) expiration of the last-to-expire valid claim of the Company’s patents covering the manufacture, use or sale or exploitation of such licensed product in such country (the Royalty Term).

The Taiho Agreement contains multiple elements, and the deliverables under the Taiho Agreement consist of (1) the research and development services, in which the Company will use commercially reasonable efforts to initiate IND enabling studies for at least five Arcus Programs, as well as further develop such Arcus Programs during the term of the Agreement, and (2) the obligation to participate on the joint steering committee. These deliverables are non-contingent in nature. The Company determined that the obligation to participate in the joint steering committee does not have stand-alone value to Taiho because the committee’s primary purpose is to monitor and govern the research and development activities and, hence, it is inseparable from the research and development services.

The Company determined that the level of effort required for it to meet its obligations under the Taiho Agreement is not expected to vary significantly over the Company’s performance period. Accordingly, the Company combined these deliverables into a single unit of accounting and allocated the entire arrangement consideration to that combined unit of accounting. As a result, the $25.0 million non-refundable, non-creditable cash payments received by the Company in 2017 is being recognized ratably over the estimated performance period of five years, the $5.0 million non-refundable, non-creditable cash payment received by the Company in 2018 is being recognized ratably over the estimated performance period of four years and the remaining $5.0 million of non-refundable, non-creditable cash payments will be recognized ratably over the estimated remaining performance period as it becomes due and payable by Taiho.

The Company also concluded that, at the inception of the agreement, Taiho’s exclusive options are substantive and that they are contingent deliverables as the exclusive options have significant uncertainty and are outside of the control of the Company, since Taiho has sole discretion to determine whether or not to exercise such options. Further, the Company concluded that the exclusive options do not contain a significant and incremental discount. In July 2018, Taiho exercised its option to the Company’s adenosine receptor antagonist program for a fee of $3.0 million, which was recognized by the Company as revenue during the year ended December 31, 2018. Upon this option exercise, Taiho now has the sole responsibility for the development and commercialization of licensed products from within the program in the Taiho Territory.

The Company also determined that the clinical and regulatory milestone payments under the Taiho Agreement do not constitute substantive milestones and, therefore, will not be accounted for under the milestone method of revenue recognition. The events leading to these payments do not meet the definition of a substantive milestone because the achievement of these events depends primarily on Taiho’s performance. Accordingly, any revenue from these payments would be recognized over the remaining period of the performance obligations, if any, relating to this arrangement. If there are no remaining performance obligations under the arrangement at the time the milestone payment is triggered, then such milestone payment will be recognized as revenue in full upon the triggering event being achieved. As of December 31, 2018, no clinical and regulatory milestones had been achieved.

During the years ended December 31, 2018 and 2017, the Company recognized a total of $8.3 million and $1.4 million, respectively, of revenue under the Taiho Agreement. As of December 31, 2018, the Company recorded as deferred revenue, current and deferred revenue, noncurrent of $6.3 million and $17.0 million, respectively, in its consolidated balance sheet. As of December 31, 2017, the Company recorded as deferred revenue, current and deferred revenue, noncurrent of $5.0 million and $18.6 million, respectively, in its consolidated balance sheet.

The Company considers the contingent payments due from Taiho upon the achievement of specified sales volumes to be similar to royalty payments. The Company will recognize royalty payments as revenue in the period when such royalty payments are earned, i.e. in the period when sales of the licensed products in Taiho Territory occur. The Taiho Agreement shall remain in effect until (a) expiration of the last exercise period if Taiho has not exercised any of its exclusive options prior to such expiration or (b) if Taiho has exercised any of its exclusive options prior to the expiration of the applicable exercise period, expiry of all Royalty Terms for the licensed products, in each case subject to certain exceptions.

WuXi Biologics License Agreement

In August 2017, the Company entered into a license agreement (the WuXi Agreement) with WuXi Biologics (Cayman) Inc. (WuXi Biologics) in which it obtained an exclusive license to develop, use, manufacture, and commercialize products including an anti-PD-1 antibody in North America, Europe, Japan and certain other territories. The Company paid upfront and milestone payments of $18.5 million during 2017 which were recorded within research and development expenses, as the products had not reached technological feasibility and did not have alternative future. No milestone payments were made during the year ended December 31, 2018. The WuXi Agreement also provides for clinical and regulatory milestone payments, commercialization milestone payments of up to $375.0 million, and tiered royalty payments to be made to WuXi Biologics that range from the high single-digits to low teens of net sales by the Company of licensed products.

Abmuno License Agreement

In December 2016, the Company entered into a license agreement (the Abmuno Agreement) with Abmuno Therapeutics LLC (Abmuno) in which it obtained a worldwide exclusive license to develop, use, manufacture, and commercialize products that include an anti-TIGIT antibody. The Company made milestone payments of $2.8 million for the year ended December 31, 2018 and upfront and milestone payments of $3.8 million for the year ended December 31, 2017, which were recorded within research and development expenses, as the products have not reached technological feasibility and do not have alternative future use and were expensed as incurred. The Abmuno Agreement also provides for additional clinical, regulatory and commercialization milestone payments of up to $101.0 million as of December 31, 2018.

Stockholders’ Deficit

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Stockholders’ Deficit

Note 7: Stockholders’ Equity (Deficit)

The Company’s Certificate of Incorporation, as amended and restated, authorizes the Company to issue 410,000,000 shares of capital stock consisting 400,000,000 shares common stock and 10,000,000 shares of preferred stock, both par value of $0.0001.

As of December 31, 2018 and 2017, the Company had reserved common stock, on an if-converted basis, for issuance as follows:

	December 31, 2018		December 31, 2017
Common stock options issued and outstanding		1,458,080		544,116
Convertible preferred stock		—		30,459,574
Remaining shares available for issuance under 2015 & 2018 Stock Plan		3,801,191		1,855,240
Total		5,259,271		32,858,930

Convertible Preferred Stock

12 Months Ended

Dec. 31, 2018

Temporary Equity Disclosure [Abstract]

Convertible Preferred Stock

Note 8: Convertible Preferred Stock

As of December 31, 2018, the Company has no outstanding convertible preferred stock. In connection with the completion of the Company’s IPO in March 2018, all outstanding shares of convertible preferred stock converted into 30,459,574 shares of common stock.

As of December 31, 2017, the outstanding convertible preferred stock was as follows (in thousands, except share amounts):

	Shares Authorized		Shares Issued and Outstanding		Liquidation Value		Carrying Value
Series A		49,725,000		12,556,791	$	49,725	$	49,637
Series B		34,653,462		8,750,852		70,000		69,817
Series C		36,580,405		9,151,931		107,000		106,742
Total		120,958,867		30,459,574	$	226,725	$	226,196

Prior to their conversion, the significant rights and preferences of the convertible preferred stock were as follows:

Dividends —The holders of the convertible preferred stock are entitled to receive dividends, out of assets legally available prior and in preference to any declaration or payment of any other dividends, at the rates of $0.24, $0.48 and $0.71 per share (as adjusted for stock splits, stock dividends, reclassifications, and the like) per annum on each outstanding share of Series A, Series B and Series C convertible preferred stock, respectively, when, as and if, declared by the board of directors. Such dividends are not cumulative. No dividends were declared or paid as of the conversion upon our IPO.

Liquidation Preference — In the event of any liquidation, dissolution, or winding up of the Company, either voluntary or involuntary, the holders of Series A, Series B and Series C preferred stock shall be entitled to receive on a pari passu basis and in preference to any distribution to the common shareholders, the greater of their stated liquidation preference or the amount such holders would have received had they converted their preferred stock into common stock immediately prior to such dissolution. For each series of convertible preferred stock, the stated liquidation preference per share is equal to $3.96, $8.00 and $11.6915 per share, respectively, plus any declared but unpaid dividends. Any remaining assets shall be distributed among the holders of common stock pro rata, based on the number of shares of common stock held by each.

Voting Rights — Each share of convertible preferred stock is entitled to one vote for each share of common stock into which such share of convertible preferred stock is convertible.

Conversion — Each share of convertible preferred stock is convertible, at the option of the holder, into the number of shares of common stock that result from dividing the applicable original share price per share by the applicable conversion price per share at the time of conversion, as adjusted for stock splits, stock dividends, reclassification and the like. At the conversion date March 15, 2018, the conversion price equaled the original share.

Stock-Based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

Note 9: Stock-Based Compensation

In May 2015, the Company adopted the 2015 Stock Plan, which was amended and restated in November 2015 (as amended from time to time, the 2015 Plan).

In March 2018, the Company adopted the 2018 Equity Incentive Plan (2018 Plan), which replaced the 2015 Plan upon completion of the IPO. 3,570,000 shares were reserved under the 2018 Plan plus 709,558 shares remaining available for issuance under the Company’s 2015 Plan and outstanding awards under its 2015 Plan that subsequently expire, lapse unexercised or are forfeited to or repurchased by the Company. As of December 31, 2018, the Company had 3,801,191 shares of common stock remain available for grant. In addition, the number of shares reserved for issuance under our 2018 Plan will automatically increase on January 1 of each year beginning January 1, 2019 by a number equal to the smallest of (i) 3,570,000 shares, (ii) 4% of the shares of common stock outstanding on the last business day of the prior fiscal year or (iii) the number of shares determined by our board of directors.

In March 2018, the Company adopted the 2018 Employee Stock Purchase Plan (2018 ESPP). The 2018 ESPP provides eligible employees with the opportunity to purchase shares of common stock through payroll deductions at a price equal to 85% of the lower of the fair market value per share on the first trading day of the applicable 24-month offering period or the fair market value per share on the applicable purchase date, provided that no more than 3,000 shares of common stock may be purchased by an employee on any purchase date. Also, the value of the shares purchased in any calendar year may not exceed $25,000. The 2018 ESPP is intended to constitute an “employee stock purchase plan” under Section 423(b) of the Internal Revenue Code of 1986, as amended. The 2018 ESPP may be terminated by the Company’s board of directors at any time. A total of 714,000 shares of common stock were initially reserved for issuance under the 2018 ESPP, and the number of shares reserved for issuance under the 2018 ESPP will automatically increase on January 1 of each year beginning on January 1, 2019 by a number of shares equal to the least of (i) 1% of our outstanding shares of common stock on the last day of the prior fiscal year, (ii) 1,071,000 shares or (iii) a number of shares determined by our board of directors.

The terms of the 2015 Plan permitted option holders to exercise stock options before they vest, subject to certain limitations. Such unvested shares are subject to repurchase by the Company at the original exercise price in the event the option holder’s service to the Company is terminated either voluntarily or involuntarily. As a result of early exercises under the 2015 Plan, approximately 927,123 and 812,769 shares had not vested and were subject to repurchase as of December 31, 2018 and 2017, respectively. The Company treats cash received from the exercise of unvested options as a refundable deposit and classifies such amounts as a liability in its consolidated balance sheets. As of December 31, 2018 and 2017, the Company included cash received for the early exercise of unvested options of $2.8 million and $0.9 million, respectively, in other current and long-term liabilities, based on the timing of their expected vesting. Amounts included in liabilities are transferred into common stock and additional paid-in capital as the shares vest, which is generally over a period of 48 months.

The following table, which includes options granted under the 2015 Plan, 2018 Plan and the Non-Plan Option, summarizes option activity:

	Shares Available for Grant			Shares Subject to Outstanding Options			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (in years)
Balance at December 31, 2015		871,226			20,200		$	0.40		8.79
Options authorized		1,133,987			—
Options granted		(746,825	)		746,825		$	0.59
Options exercised		—			(637,107	)	$	0.48
Options repurchased		24,987			—		$	0.40
Balance at December 31, 2016		1,283,375			129,918		$	1.20		9.84
`Options authorized		1,568,397			—
Options granted		(1,063,019	)		1,063,019		$	1.61
Options exercised		—			(643,024	)	$	1.45
Options forfeited or canceled		5,797			(5,797	)	$	1.28
Options repurchased		60,690			—		$	0.49
Balance at December 31, 2017		1,855,240			544,116		$	1.71		9.28
Options authorized		3,570,000
Options granted		(1,645,489	)		1,645,489		$	8.19
Options exercised		—			(720,756	)	$	4.64
Options forfeited or canceled		10,769			(10,769	)	$	5.25
Options repurchased		10,671
Balance at December 31, 2018		3,801,191			1,458,080		$	7.55		8.99
Options outstanding and exercisable as of December 31, 2018					238,721		$	6.49		8.87
Options vested and expected to vest as of December 31, 2018					1,458,080		$	7.55		8.99

The following table summarizes employee and non-employee stock-based compensation expense for the years ended December 31, 2018, 2017 and 2016, and also the allocation within the consolidated statements of operations and comprehensive loss (in thousands):

	Year Ended December 31, 2018		Year Ended December 31, 2017		Year Ended December 31, 2016
Research and development	$	2,255	$	222	$	67
General and administrative		1,619		273		23
Total stock-based compensation	$	3,874	$	495	$	90

The Company estimates the fair value of options and ESPP shares utilizing the Black-Scholes option pricing model, which is dependent upon several variables, such as expected term, volatility, risk-free interest rate, and expected dividends. Each of these inputs is subjective and generally requires significant judgment to determine. Stock-based compensation is measured at the grant date based on the fair value of the award and is recognized as expense, net of forfeitures, over the requisite service period, which is generally the vesting period of the respective award. The Company recognizes compensation on a straight-line basis over the requisite vesting period for each award. The following assumptions were used to calculate the fair value of stock-based compensation as of December 31, 2018, 2017 and 2016:

	Stock Options			ESPP
	Year Ended December 31, 2018	Year Ended December 31, 2017	Year Ended December 31, 2016	Year Ended December 31, 2018
Risk-free interest rate	1.2% - 3.1%	1.66% - 2.20%	1.2% - 2.45%	2.11% - 2.63%
Expected term (in years)	5.16-9.95	5.95-9.99	6.25-9.84	.5-2.0
Volatility	58.7%-75.5%	67.0%-71.7%	67.0 - 77.8%	54.3% - 65.5%
Dividend yield	0%	0%	0%	0%

Expected Term — The Company has opted to use the “simplified method” for estimating the expected term of options, whereby the expected term equals the arithmetic average of the vesting term and the original contractual term of the option (generally 10 years).

Expected Volatility — Due to the Company’s limited operating history and a lack of company specific historical and implied volatility data, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. The historical volatility data was computed using the daily closing prices for the selected companies’ shares during the equivalent period of the calculated expected term of the stock-based awards.

Risk-Free Interest Rate — The risk-free rate assumption is based on the U.S. Treasury instruments with maturities similar to the expected term of the Company’s stock options.

Expected Dividend — The Company has not issued any dividends in its history and does not expect to issue dividends over the life of the options and therefore has estimated the dividend yield to be zero.

Fair value of Common Stock — The fair value of the shares of common stock underlying the stock-based awards has historically been determined by the board of directors, with input from management. Because there has been no public market for the Company’s common stock, the board of directors has determined the fair value of the common stock on the grant-date of the stock-based award by considering a number of objective and subjective factors, including enterprise valuations of the Company’s common stock performed by an unrelated third-party specialist, valuations of comparable companies, sales of the Company’s convertible preferred stock to unrelated third parties, operating and financial performance, the lack of liquidity of the Company’s capital stock, and general and industry-specific economic outlook. The board of directors intended all options granted to be exercisable at a price per share not less than the estimated per share fair value of common stock underlying those options on the date of grant.

As of December 31, 2018, 2017 and 2016, there was a total of $11.0 million and $1.9 million, $0.4 million, respectively, of unrecognized employee compensation costs related to non-vested stock option awards. During the years ended December 31, 2018, 2017 and 2016, the intrinsic value of shares exercised was $0.6 million, $2.5 million and $1.2 million, respectively, and the fair value of shares vested during the respective years was $3.0 million, $0.5 million and $0.1 million.

Non-employee stock-based compensation

As of December 31, 2018, 2017 and 2016, 14,918, 63,878 and 37,311, respectively, of vested stock options and 31,388, 77,466 and 47,971, respectively, of unvested stock options were held by non-employees. The Company remeasures the estimated fair value of the unvested portion of the award each period, until the award is fully vested. The Company believes that the fair value of the stock options is more reliably measurable than the fair value of services received. The fair value of options granted to non-employees was estimated using the Black-Scholes method. The amount of stock-based compensation expense related to non-employees recognized in the consolidated financial statements for the year ended December 31, 2018 was $0.3 million and for the years ended December 31, 2017 and 2016 was immaterial.

Restricted stock awards

In 2015, in conjunction with the incorporation of the Company, the Company issued a total of 2,777,776 shares of common stock at $0.0004 per share to its two founders, the Chief Executive Officer and the President, under restricted stock agreements. At the date of grant, the shares had an estimated fair value of $0.0004 per share. Under the terms of the restricted stock agreements, shares vest monthly over four years. Upon the termination of service of these individuals, unvested shares are subject to repurchase by the Company at the original issue price.

A summary of the Company’s non-vested restricted stock for the periods presented is as follows:

	Number of Shares		Weighted Average Grant Date Fair Value		Remaining Contractual Term (Years)
Balance, December 31, 2015		2,372,684	$	0.000396		3.4
Vested during the year		694,444	$	0.000396
Balance, December 31, 2016		1,678,240	$	0.000396		2.4
Vested during the year		694,444	$	0.000396
Balance, December 31, 2017		983,796	$	0.000396		1.4
Vested during the year		694,444	$	0.000396
Balance, December 31, 2018		289,352	$	0.000396		0.4

Net Loss per Share

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Net Loss per Share

Note 10. Net Loss per Share

The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per share data):

	Year Ended December 31, 2018			Year Ended December 31, 2017			Year Ended December 31, 2016
Numerator:
Net loss	$	(49,594	)	$	(53,082	)	$	(17,970	)
Denominator:
Weighted-average common shares outstanding		36,357,336			3,885,508			3,374,609
Less: weighted-average common shares subject to repurchase		(1,739,099	)		(2,057,246	)		(2,510,626	)
Weighted-average common shares used to compute basic and diluted net loss per share		34,618,237			1,828,262			863,983
Net loss per share, basic and diluted	$	(1.43	)	$	(29.03	)	$	(20.80	)

The following outstanding potentially dilutive securities were excluded from the computation of diluted net loss per share for the periods presented because including them would have been antidilutive:

	December 31, 2018		December 31, 2017		December 31, 2016
Convertible preferred stock		—		30,459,579		21,307,643
Common stock options issued and outstanding		1,458,079		544,116		129,918
Unvested restricted common stock		289,352		983,796		1,678,240
Unvested early exercised common stock options		927,123		812,769		525,797
Total		2,674,554		32,800,260		23,641,598

Provision for Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Provision for Income Taxes

Note 11: Provision for Income Taxes

The provision for income taxes differs from the amount expected by applying the federal statutory rate to the loss before taxes as follows:

	Year Ended December 31, 2018			Year Ended December 31, 2017			Year Ended December 31, 2016
Federal statutory income tax rate		21.00	%		34.00	%		34.00	%
Non-deductible expenses and other		(1.54	)%		(1.23	)%		(0.22	)%
Change in valuation allowance		(19.46	)%		(16.46	)%		(33.78	)%
Remeasurement of federal tax rate change		—			(16.32	)%		—
Total		0.00	%		0.00	%		0.00	%

As of December 31, 2018 and 2017, the components of the Company’s deferred tax assets are as follows (in thousands):

	Year Ended December 31, 2018			Year Ended December 31, 2017
Deferred tax assets:
Federal and state net operating loss carryforwards	$	16,076		$	11,101
Research and development credits carryforwards		5,087			2,706
Depreciation		3,942			3,489
Deferred Revenue		3,909			—
Other		1,724			1,292
Total deferred tax assets		30,738			18,588
Less valuation allowance		(30,738	)		(18,588	)
Net deferred tax assets	$	—		$	—

Deferred income taxes reflect the net tax effects of (a) temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes, and (b) operating losses and tax credit carryforwards.

The Company’s accounting for deferred taxes involves the evaluation of a number of factors concerning the realizability of its net deferred tax assets. The Company primarily considered such factors as its history of operating losses, the nature of the Company’s deferred tax assets, and the timing, likelihood and amount, if any, of future taxable income during the periods in which those temporary differences and carryforwards become deductible. At present, the Company does not believe that it is more likely than not that the deferred tax assets will be realized; accordingly, a full valuation allowance has been established and no deferred tax asset is shown in the accompanying consolidated balance sheets. The valuation allowance increased by approximately $12.2 million, 10.2 million and $7.6 million, respectively, for the years ended December 31, 2018, 2017 and 2016.

In December 2017, the 2017 Tax Cuts and Jobs Act (2017 Tax Act) was enacted and includes a broad range of provisions, many of which differ significantly from those contained in previous U.S. tax law. Changes in tax law are accounted for in the period of enactment. As such, the Company’s consolidated financial statements as of December, 31, 2018 and 2017 reflect the impact of this 2017 Tax Act, which primarily consisted of measuring the Company’s deferred tax assets and valuation allowance using the newly enacted U.S. corporate tax rate.

At December 31, 2018, the Company has total net operating loss carryforwards (NOLs) of $70.9 million for federal income tax purposes, of which approximately $47.4 million begin to expire in 2035 and approximately $23.6 million that have no expiration date and federal research tax credits of approximately $3.6 million that begin to expire in 2035. The Company also has state NOLs of approximately $15.4 million that begin to expire in 2035, and state research tax credits of approximately $3.1 million that have no expiration date. Use of the NOLs and credit carryforwards may be subject to a substantial annual limitation due to the ownership change provisions of U.S. tax law, as defined in Section 382 and 383 of the Internal Revenue Code of 1986, as amended, and similar state provisions. The annual limitation may result in the expiration of NOLs and credits before use. The Company determined that an ownership change occurred in 2015 in conjunction with its Series A Preferred Stock financing and in 2017 in conjunction with its Series C Preferred Stock financing but does not expect that these ownership changes will result in the expiration of any the NOLs prior to utilization.

The Company has not been audited by the Internal Revenue Service, any state or foreign tax authority. The Company is subject to taxation in the United States and also beginning in 2017, in Australia. Because of the net operating loss and research credit carryforwards, all of the Company’s tax years, from 2015 to 2018, remain open to U.S. federal and California state tax examinations. In addition, the Company’s tax years from 2017 to 2018 are open to examination in Australia. There were no interest or penalties accrued at December 31, 2018, 2017 and 2016.

Uncertain Tax Positions

The Company follows the provisions of FASB Accounting Standards Codification (ASC 740-10), Accounting for Uncertainty in Income Taxes . ASC 740-10 prescribes a comprehensive model for the recognition, measurement, presentation and disclosure in financial statements of uncertain tax positions that have been taken or expected to be taken on a tax return. No liability related to uncertain tax positions is recorded in the consolidated financial statements. The Company’s reserve for unrecognized tax benefits is approximately $1.1 million and $0.6 million and $0.2 million at December 31, 2018, 2017 and 2016, respectively.

Due to the full valuation allowance at December 31, 2018, 2017 and 2016, current adjustments to the unrecognized tax benefit will have no impact on the Company’s effective income tax rate; any adjustments made after the valuation allowance is released will have an impact on the tax rate.

A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands):

	Year Ended December 31, 2018		Year Ended December 31, 2017		Year Ended December 31, 2016
Beginning balance	$	622	$	242	$	19
Additions for tax positions taken in a prior year		8		29		—
Additions for tax positions taken in current year		454		351		223
Ending balance	$	1,084	$	622	$	242

The Company does not anticipate material changes to its uncertain tax positions through the next 12 months.

Commitments

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments

Note 12: Commitments

Purchase Commitments

The Company has contractual arrangements with research and development organizations and suppliers; however, these contracts are generally cancelable on 30 days’ notice and the obligations under these contracts are largely based on services performed.

Leases

The Company leases office space in Hayward, California under non-cancelable operating leases with expiration in 2025. Rent expense was $1.6 million, $0.9 million and $0.6 million for the years ended December 31, 2018, 2017 and 2016, respectively. Future minimum lease payments under non-cancelable operating leases as of December 31, 2018 are as follows (in thousands):

Year ending December 31:	Operating Leases
2019	$	2,041
2020		2,105
2021		2,195
2022		2,265
2023		2,339
2024 and beyond		4,487
Total	$	15,432

Total minimum lease payments have not been reduced by minimum sublease rent income of approximately $0.6 million under a future noncancelable sublease.

The Company has provided deposits for letters of credit totaling $0.2 million to secure its obligations under its lease, which have been classified as long-term assets on the Company’s consolidated balance sheet as of December 31, 2018.

Indemnification

As permitted under Delaware law and in accordance with the Company’s bylaws, the Company is required to indemnify its officers and directors for certain events or occurrences while the officer or director is or was serving in such capacity. The Company is also party to indemnification agreements with its directors and officers. The Company believes the fair value of the indemnification rights and agreements is minimal. Accordingly, the Company has not recorded any liabilities for these indemnification rights and agreements as of December 31, 2018, 2017 and 2016.

Employee Benefit Plan

12 Months Ended

Dec. 31, 2018

Compensation And Retirement Disclosure [Abstract]

Employee Benefit Plan

Note 13: Employee Benefit Plan

The Company sponsors a 401(k) defined contribution plan for its employees. This plan provides for tax-deferred salary deductions for all employees. Employee contributions are voluntary. Employees may contribute up to 100% of their annual compensation to this plan, as limited by an annual maximum amount as determined by the Internal Revenue Service. The Company may match employee contributions in amounts to be determined at the Company’s sole discretion. The Company made no contributions to the plan for the years ended December 31, 2018, 2017 and 2016.

Selected Unaudited Quarterly Financial Data

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Selected Unaudited Quarterly Financial Data

Note 14: Selected Unaudited Quarterly Financial Data

The following table summarizes the Company’s unaudited quarterly financial data for the last two years (in thousands, except per share data):

	First Quarter			Second Quarter			Third Quarter			Fourth Quarter
2018
Total revenues	$	1,250		$	1,250		$	4,291		$	1,562
Total operating expenses	$	14,581		$	17,149		$	16,436		$	15,046
Net loss	$	(12,954	)	$	(13,533	)	$	(10,812	)	$	(12,295	)
Net loss per share — basic and diluted	$	(1.37	)	$	(0.32	)	$	(0.25	)	$	(0.28	)
Weighted average number of shares, basic and diluted		9,488,352			42,533,641			42,838,098			43,163,412
2017
Total revenues	$	—		$	—		$	163		$	1,250
Total operating expenses	$	7,300		$	9,664		$	23,297		$	14,594
Net loss	$	(7,200	)	$	(9,550	)	$	(23,122	)	$	(13,210	)
Net loss per share — basic and diluted	$	(4.96	)	$	(5.64	)	$	(11.86	)	$	(5.98	)
Weighted average number of shares, basic and diluted		1,452,215			1,693,150			1,949,258			2,208,065

Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The consolidated financial statements and accompanying notes have been prepared in accordance with generally accepted accounting principles in the United States of America (U.S. GAAP) and include all adjustments necessary for the fair presentation of the Company’s financial position for the periods presented.
Principles of Consolidation	Principles of Consolidation During 2017, the Company established a wholly-owned subsidiary in Australia. The consolidated financial statements include the Company’s accounts and those of its wholly-owned subsidiary. All intercompany accounts, transactions and balances have been eliminated.
Reverse Stock Split	Reverse Stock Split On March 9, 2018, the Company effected a reverse split of all shares of its common and preferred stock at a ratio of 1-for-3.96 (the Reverse Split). The par values and the authorized shares of the common and preferred stock were not adjusted as a result of the Reverse Split. All references to shares of common stock outstanding, average number of shares outstanding and per share amounts in these consolidated financial statements and notes to the consolidated financial statements have been adjusted within the consolidated financial statements, on a retroactive basis, to reflect the Reverse Split.
Use of Estimates	Use of Estimates The preparation of the Company’s consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, as well as related disclosure of contingent assets and liabilities. Estimates were used to determine the fair value of common stock prior to the IPO and are used to determine stock-based awards and other issuances, accruals for research and development costs, useful lives of long-lived assets, and uncertain tax positions. Actual results could differ materially from the Company’s estimates.
Risks And Uncertainties	Risk and Uncertainties The Company’s future results of operations involve a number of risks and uncertainties. Factors that could affect the Company’s future operating results and cause actual results to vary materially from expectations include, but are not limited to, uncertainty of results of clinical trials and reaching milestones, uncertainty of regulatory approval of the Company’s potential drug candidates, uncertainty of market acceptance of the Company’s product candidates, competition from substitute products and larger companies, securing and protecting proprietary technology, strategic relationships and dependence on key individuals and sole source suppliers. The Company’s product candidates require approvals from the U.S. Food and Drug Administration (FDA) and comparable foreign regulatory agencies prior to commercial sales in their respective jurisdictions. There can be no assurance that any product candidates will receive the necessary approvals. If the Company does not obtain approval and does not successfully commercialize any of its product candidates, it would have a materially adverse impact on the Company.
Segments	Segments The Company operates and manages its business as one reportable and operating segment, which is the business of developing and commercializing immunotherapies. The Company’s chief executive officer, who is the chief operating decision maker, reviews financial information on an aggregate basis for purposes of allocating and evaluating financial performance. All long-lived assets are maintained in the United States of America.
Cash Equivalents, Short-Term and Long-Term Investments	Cash Equivalents, Short-Term and Long-Term Investments Cash equivalents include marketable securities having an original maturity of three months or less at the time of purchase. Short-term investments have maturities of greater than three months at the time of purchase. Long-term investments have maturities greater than 12 months at the time of purchase. Collectively, cash equivalents, short-term and long-term investments are considered available-for-sale and are recorded at fair value. Unrealized gains and losses are recorded in accumulated other comprehensive loss. Realized gains and losses are included in interest and other income, net in the consolidated statements of operations and comprehensive loss. The basis on which the cost of a security sold or amount reclassified out of accumulated other comprehensive income into earnings is determined using the specific identification method.
Restricted Cash	Restricted Cash Restricted cash at December 31, 2018 and 2017 comprises cash balances primarily held as security in connection with the Company’s facility lease agreement and is included in long-term assets in the consolidated balance sheets.
Receivable From a Related Party	Receivable From a Related Party Receivable from a related party is recorded net of any allowances. As of December 31, 2018 and 2017, the outstanding amount is due from PACT Pharma, Inc. (PACT Pharma) for expenses the Company paid for on its behalf. The Company is exposed to credit risk in the event of a default by PACT Pharma. To date, the Company has not experienced any losses related to these receivables (see Note 5) and has not recorded any allowances.
Fair Value Measurements	Fair Value Measurements Fair value accounting is applied for all financial assets and liabilities, including short-term and long-term investments, and non-financial assets and liabilities that are recognized or disclosed at fair value in the consolidated financial statements on a recurring basis (at least annually). The carrying amount of the Company’s financial instruments, including receivable from a related party, accounts payable and accrued expenses and other current liabilities approximate fair value due to their short-term maturities.
Concentration of Credit Risk	Concentration of Credit Risk Cash equivalents, short-term and long-term investments are financial instruments that potentially subject the Company to concentrations of credit risk. The Company invests in money market funds, treasury bills and notes, government bonds, commercial paper and corporate notes. The Company limits its credit risk associated with cash equivalents, short-term and long-term investments by placing them with banks and institutions it believes are highly credit worthy and in highly rated investments.
Property and Equipment	Property and Equipment Property and equipment are stated at cost and depreciated using the straight-line method over the estimated useful lives of the assets, ranging from one to five years. Leasehold improvements are amortized over the shorter of their estimated useful lives or the related lease term. Upon retirement or sale, the cost and related accumulated depreciation are removed from the consolidated balance sheet and the resulting gain or loss is reflected in the consolidated statement of operations and comprehensive loss.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company reviews long-lived assets, including property and equipment, for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. An impairment charge would be recorded when estimated undiscounted future cash flows expected to result from the use of the asset and its eventual disposition are less than its carrying amount. Impairment, if any, is assessed using discounted cash flows or other appropriate measures of fair value. The Company did not recognize any impairment charges for the years ended December 31, 2018 and 2017.
Deferred Offering Costs	Deferred Offering Costs Deferred offering costs associated with the Company’s IPO, consisting of legal, accounting, filing and other fees directly related to the IPO, were capitalized. The deferred offering costs, which totaled $3.6 million, were reclassified to additional paid-in capital upon the effectiveness of the IPO in March 2018. As of December 31, 2017, $1.3 million of deferred offering costs were capitalized and included in other long-term assets in the consolidated balance sheet.
Revenue Recognition	Revenue Recognition The Company generates revenue from its option and license agreement for the development and commercialization of its product candidates. Option and license agreements may include non-refundable upfront research and development fees, option fees to obtain development and commercialization licenses for the Company’s products, milestone payments based on achievement of defined development, regulatory and sales targets, and royalties on sales of commercialized products. To date, the Company has not recognized revenue from sales of its product candidates. The Company recognizes revenue when all four of the following criteria have been met: (i) collectability is reasonably assured; (ii) delivery has occurred or services have been rendered; (iii) persuasive evidence of an arrangement exists; and (iv) the fee is fixed or determinable. Revenue under option and license arrangements is recognized based on evaluation of the performance obligations of the contract. Collectability is assessed based on evaluation of payment criteria as stated in the contract as well as the creditworthiness of the customer. Determination of whether delivery has occurred, or services rendered are based on management’s evaluation of the performance obligations as stated in the contract and progress made against those obligations. Evidence of an arrangement is deemed to exist upon execution of the contract. Fees are considered fixed and determinable when the amount payable to the Company is no longer subject to any acceptance, refund rights or other contingencies that would alter the fixed nature of the fees charged for the deliverables. Option and license agreements may contain multiple elements as evaluated under Accounting Standards Codification (ASC) 605-25, Revenue Recognition- Multiple-Element Arrangements, including agreements to provide research and development services, participation in development and/or steering committees, manufacturing services, sharing of know-how and other information, and grants of licenses to develop and commercialize product candidates. Each deliverable under the agreement is evaluated to determine whether it qualifies as a separate unit of accounting based on whether the deliverable has standalone value to the customer. The arrangement’s consideration that is fixed or determinable is then allocated to each separate unit of accounting based on the following hierarchy: (i) vendor-specific objective evidence of the fair value of the deliverable, if it exists; (ii) third-party evidence of selling price, if vendor-specific objective evidence is not available; or (iii) the best estimate of selling price if neither vendor-specific objective evidence or third-party evidence is available. A delivered item or items that do not qualify as a separate unit of accounting within the arrangement are combined with the other applicable undelivered items within the arrangement. The allocation of arrangement consideration and the recognition of revenue is then determined for those combined deliverables as a single unit of accounting. For a combined unit of accounting, non-refundable upfront fees are recognized as performance obligations related to the final deliverable are completed. In the case of research and development services, performance would generally be expected to be ratable over the estimated performance period unless the Company determines there is a discernible pattern of performance other than straight-line, in which case the Company uses a proportionate performance method to recognize the revenue over the estimated performance period. Amounts received in advance of performance are recorded as deferred revenue. If any of the initial deliverables are determined to have standalone value separate from the research and development services, then the allocated consideration is recorded as revenue when those items are delivered. Options to acquire development and commercialization licenses of the Company’s products are evaluated to determine if they are substantive. Fees for substantive options are recognized as revenue when an option is exercised by the collaboration partner and the Company has completed the deliverables that are associated with exercise of such option. Option and license agreements may also contain milestone payments that become due upon the achievement of certain milestones. The Company applies ASC 605-28, Revenue Recognition—Milestone Method. Under the milestone method, payments that are contingent upon achievement of a substantive milestone are recognized in the period in which the milestone is achieved. Milestones are defined as an event that can only be achieved based on the Company’s performance and there is substantive uncertainty about whether the event will be achieved at the inception of the arrangement. Events that are contingent only on the passage of time or only on counterparty performance are not considered milestones subject to this guidance. Further, for the milestone to be considered substantive, the amounts received must relate solely to prior performance, be reasonable relative to all of the deliverables, and the consideration must be commensurate with the Company’s performance to achieve the milestone. Non-substantive milestone payments are recognized as revenue over the estimated period of any remaining performance obligations.
Research and Development Expenses	Research and Development Expenses Research and development costs are expensed as incurred. Research and development expenses consist primarily of personnel costs for the Company’s research and product development employees. Also included are non-personnel costs such as professional fees payable to third parties for preclinical and clinical studies and research services, laboratory supplies and equipment maintenance, product licenses, and other consulting costs. The Company estimates preclinical and clinical study and research expenses based on the services performed, pursuant to contracts with research institutions that conduct and manage preclinical and clinical studies and research services on its behalf. The Company estimates these expenses based on discussions with internal management personnel and external service providers as to the progress or stage of completion of services and the contracted fees to be paid for such services. If the actual timing of the performance of services or the level of effort varies from the original estimates, the Company will adjust the accrual accordingly. Payments associated with licensing agreements to acquire exclusive licenses to develop, use, manufacture and commercialize products that have not reached technological feasibility and do not have alternative future use are expensed as incurred. Payments made to third parties under these arrangements in advance of the performance of the related services by the third parties are recorded as prepaid expenses until the services are rendered.
Leases and Rent Expense	Leases and Rent Expense The Company records rent expense on a straight-line basis over the life of the lease. In cases where there is a free rent period or future fixed rent escalations, the Company records a deferred rent liability. Additionally, the receipt of any lease incentives is recorded as a deferred rent liability which is amortized over the lease term as a reduction of rent expense. Any lease incentives that are due from the landlord but have not been collected are recorded as a receivable in Prepaid expenses and other current assets. Building improvements made with the lease incentives or tenant allowances are capitalized as leasehold improvements and included in property and equipment in the consolidated balance sheets.
Stock-Based Compensation	Stock-Based Compensation The Company accounts for stock-based compensation arrangements with employees in accordance with ASC 718, Stock Compensation. Stock-based awards granted include stock options with time-based vesting. ASC 718 requires the recognition of compensation expense, using a fair value-based method, for costs related to all stock-based payments. The Company’s determination of the fair value of stock options with time-based vesting on the date of grant utilizes the Black-Scholes option-pricing model, and is impacted by its common stock price as well as other variables including, but not limited to, expected term that options will remain outstanding, expected common stock price volatility over the term of the option awards, risk-free interest rates and expected dividends. The fair value of a stock-based award is recognized over the period during which an optionee is required to provide services in exchange for the option award, known as the requisite service period (usually the vesting period) on a straight-line basis. Stock-based compensation expense is recognized based on the fair value determined on the date of grant and is reduced for forfeitures as they occur. Equity instruments issued to non-employees are accounted for in accordance with ASC 505-50, Equity Based Payments to Non-Employees, and are recorded at their fair value on the measurement date and are subject to periodic adjustments as the underlying equity instruments vest. The fair value of options granted to consultants is expensed when vested. Non-employee stock-based compensation expense was not material for all periods presented. Estimating the fair value of equity-settled awards as of the grant date using valuation models, such as the Black-Scholes option pricing model, is affected by assumptions regarding a number of complex variables. Changes in the assumptions can materially affect the fair value and ultimately how much stock-based compensation expense is recognized. These inputs are subjective and generally require significant analysis and judgment to develop.
Convertible Preferred Stock	Convertible Preferred Stock The Company records all shares of convertible preferred stock at their respective fair values less issuance costs on the dates of issuance. The convertible preferred stock is recorded outside of stockholders’ equity (deficit) because, in the event of certain deemed liquidation events considered not solely within the Company’s control, such as a merger, acquisition and sale of all or substantially all of the Company’s assets, the convertible preferred stock will become redeemable at the option of the holders. In the event of a change of control of the Company, proceeds received from the sale of such shares will be distributed in accordance with the liquidation preferences set forth in the Company’s Amended and Restated Certificate of Incorporation unless the holders of convertible preferred stock have converted their shares of convertible preferred stock into shares of common stock. The Company has determined not to adjust the carrying values of the convertible preferred stock to the liquidation preferences of such shares because of the uncertainty of whether or when such an event would occur. All of outstanding convertible preferred stock converted into common stock in March 2018 upon the effectiveness of the IPO.
Income Taxes	Income Taxes The Company provides for income taxes under the asset and liability method. Current income tax expense or benefit represents the amount of income taxes expected to be payable or refundable for the current year. Deferred income tax assets and liabilities are determined based on differences between the financial statement reporting and tax bases of assets and liabilities and net operating loss and credit carryforwards, and are measured using the enacted tax rates and laws that will be in effect when such items are expected to reverse. Deferred income tax assets are reduced, as necessary, by a valuation allowance when management determines it is more likely than not that some or all of the tax benefits will not be realized. The Company accounts for uncertain tax positions in accordance with ASC 740-10, Accounting for Uncertainty in Income Taxes. The Company assesses all material positions taken in any income tax return, including all significant uncertain positions, in all tax years that are still subject to assessment or challenge by relevant taxing authorities. Assessing an uncertain tax position begins with the initial determination of the position’s sustainability and is measured at the largest amount of benefit that is greater than fifty percent likely of being realized upon ultimate settlement. As of each balance sheet date, unresolved uncertain tax positions must be reassessed, and the Company will determine whether (i) the factors underlying the sustainability assertion have changed and (ii) the amount of the recognized tax benefit is still appropriate. The recognition and measurement of tax benefits requires significant judgment. Judgments concerning the recognition and measurement of a tax benefit might change as new information becomes available. The Company includes any penalties and interest expense related to income taxes as a component of other expense and interest expense, net, as necessary.
Comprehensive Loss	Comprehensive Loss Comprehensive loss is defined as a change in equity of a business enterprise during a period, resulting from transactions from non-owner sources. The Company had unrealized loss from its available-for-sale securities during the years ended December 31, 2018, 2017 and 2016, which meets the criteria as other comprehensive loss and, therefore, the Company has reported comprehensive loss and net loss.
Net Loss per Share	Net Loss per Share Basic net loss per share is calculated by dividing the net loss by the weighted-average number of common shares outstanding during the period, without consideration for potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss by the weighted-average number of common shares and potentially dilutive securities outstanding for the period. For purposes of the diluted net loss per share calculation, convertible preferred stock, and common stock options are considered to be potentially dilutive securities. Because the Company reported a net loss for the years ended December 31, 2018, 2017 and 2016, and the inclusion of the potentially dilutive securities would be antidilutive, diluted net loss per share is the same as basic net loss per share for all periods.
Recent Accounting Pronouncements	Recent Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (FASB), or other standard setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the impact of recently issued standards that are not yet effective will not have a material impact on the Company’s consolidated financial statements upon adoption. Under the Jumpstart Our Business Startups Act of 2012, as amended (the JOBS Act), the Company meets the definition of an emerging growth company, and has elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act. The Company did not adopt any new accounting standards during 2018.
Recently Issued Accounting Standards or Updates Not Yet Effective	Recently Issued Accounting Standards or Updates Not Yet Effective In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (ASU 2014-09). The FASB issued numerous updates that provide clarification on a number of specific issues as well as requiring additional disclosures. Collectively, the new revenue standards became effective for public business entities for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, ASU 2014-09 will be effective for the Company for the year ended December 31, 2019, and all interim periods thereafter. Early adoption is permitted. ASU 2014-09 also permits two methods of adoption: retrospectively to each prior reporting period presented (full retrospective method), or retrospectively with the cumulative effect of initially applying the guidance recognized at the date of initial application (the modified retrospective method). The core principle of ASU 2014-09 is that an entity should recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. ASU 2014-09 defines a five-step process to achieve this core principle and, in doing so, it is possible more judgment and estimates may be required within the revenue recognition process than required under existing U.S. generally accepted accounting pronouncements. The Company has substantially completed its evaluation related to the adoption of ASU 2014-09, applying the five-step model of the new standard to the option and license agreement with Taiho Pharmaceutical Co., Ltd. (Taiho), the only agreement which will be impacted by the adoption of the new standard, specifically as it pertains to the non-refundable, non-creditable upfront cash payments to the Company totaling $35.0 million, the option exercise payment of $3.0 million and all other future contingent payments the Company may become entitled to. The Company will finalize its accounting assessment and quantitative impact of the adoption during the first quarter of fiscal year 2019, using the modified retrospective method, which will reflect the cumulative effect of the adoption retrospectively as of January 1, 2019, the initial date of adoption. In January 2016, the FASB issued ASU No. 2016-01 (Subtopic 825-10), Financial Instruments (ASU 2016-01). ASU 2016-01 requires management to measure marketable investments at fair value with changes in fair value recognized in net income or loss. ASU 2016-01 will impact the disclosure and presentation of financial assets and liabilities. ASU 2016-01 was effective for public business entities for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, ASU 2016-01 will be effective for the Company as of December 31, 2019, and all interim periods within. Early adoption is not permitted. The adoption of this standard is not expected to have a material impact on the Company’s consolidated financial statements. In February 2016, the FASB issued ASU No. 2016-02 (Topic 842), Leases (ASU 2016-02). ASU 2016-02 requires an entity to recognize assets and liabilities arising from a lease for both financing and operating leases. The ASU will also require new qualitative and quantitative disclosures to help investors and other financial statement users better understand the amount, timing, and uncertainty of cash flows arising from leases. For public entities, ASU 2016-02 is effective for fiscal years beginning after December 15, 2018. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, ASU 2016-02 is effective for the Company for the year ended December 31, 2020, and all interim periods within. Early adoption is permitted. The Company has not yet determined the potential effects of this ASU on its consolidated financial statements. In November 2016, the FASB issued ASU No. 2016-18 (Topic 230), Restricted Cash, Statement of Cash Flows (ASU 2016-18). ASU 2016-18 requires that a statement of cash flows explain the change during the period in the total of cash and, cash equivalents, and amounts generally described as restricted cash or restricted cash equivalents. Therefore, amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the consolidated statement of cash flows. ASU 2016-18 was effective for public business entities for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, ASU 2016-18 will be effective for the Company for the year ended December 31, 2019, and all interim periods thereafter. Early adoption is permitted. The amendments in this ASU should be applied using a retrospective transition method to each period presented. The adoption of this standard is not expected to have a material impact on the Company’s consolidated financial statements. In June 2018, the FASB issued ASU No. 2018-07 (Topic 718), Compensation – Stock Compensation (ASU 2018-07). ASU 2018-07 requires an entity to expand the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from nonemployees. For public entities, ASU 2018-07 is effective for fiscal years beginning after December 15, 2018. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, ASU 2018-07 is effective for the Company for the year ended December 31, 2020, and all interim periods within. Early adoption is permitted. The Company has not yet determined the potential effects of this ASU on its consolidated financial statements. In August 2018, the FASB issued ASU No.2018-13 (Topic 820), Fair Value Measurement. ASU 2018-13 modifies the disclosure requirements on fair value measurement in Topic 820. For public entities, ASU 2018-013 is effective for fiscal years beginning after December 15, 2019. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, ASU 2018-13 is effective for the Company for the year ended December 31, 2020, and all interim periods within. Early adoption is permitted. The adoption of this standard is not expected to have a material impact on the Company’s consolidated financial statements.

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Schedule of Fair Value, Assets Measured on Recurring Basis

The following tables set forth the Company’s financial instruments that were measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands):

	Fair Value Measurements at December 31, 2018
	Total		Level 1		Level 2		Level 3
Assets
Money market funds	$	45,017	$	45,017	$	—	$	—
U.S. government agency obligations		103,940		—		103,940		—
Corporate securities and commercial paper		110,768		—		110,768		—
	$	259,725	$	45,017	$	214,708	$	—

	Fair Value Measurements at December 31, 2017
	Total		Level 1		Level 2		Level 3
Assets
Money market funds	$	66,478	$	66,478	$	—	$	—
U.S. government agency obligations		57,153		—		57,153		—
Corporate securities and commercial paper		52,072		—		52,072		—
	$	175,703	$	66,478	$	109,225	$	—

Schedule of Investments Classified as Available for Sale Securities with Contractual Maturities

Classified as (with contractual maturities):

	December 31, 2018		December 31, 2017
Cash and cash equivalents	$	71,064	$	98,426
Short-term investments (due within one year)		185,480		77,277
Long-term investments (due between one and two years)		3,181		—
	$	259,725	$	175,703

Schedule of Available for Sale Securities at Fair Value Measurements

	Fair Value Measurements at December 31, 2018
	Amortized Cost		Unrealized Gain		Unrealized Loss			Fair Value
Money market funds	$	45,017	$	—	$	—		$	45,017
U.S. government agency obligations		103,957		—		(17	)		103,940
Corporate securities and commercial paper		110,859		—		(91	)		110,768
	$	259,833	$	—	$	(108	)	$	259,725

	Fair Value Measurements at December 31, 2017
	Amortized Cost		Unrealized Gain		Unrealized Loss			Fair Value
Money market funds	$	66,478	$	—	$	—		$	66,478
U.S. government agency obligations		57,183		—		(30	)		57,153
Corporate securities and commercial paper		52,084		—		(12	)		52,072
	$	175,745	$	—	$	(42	)	$	175,703

Consolidated Balance Sheet Components (Tables)

12 Months Ended

Dec. 31, 2018

Balance Sheet Related Disclosures [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):

	As of December 31, 2018			As of December 31, 2017
Scientific equipment	$	6,628		$	5,053
Furniture and equipment		813			625
Capitalized software		146			131
Leasehold improvements		10,828			9,280
Construction in progress		335			119
Total		18,750			15,208
Less: Accumulated depreciation and amortization		(7,643	)		(3,978	)
Property and equipment, net	$	11,107		$	11,230

Summary of Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):

	As of December 31, 2018		As of December 31, 2017
Accrued personnel expenses	$	2,833	$	1,026
Accrued research and development expenses		2,816		1,193
Professional fees		211		706
Other		163		212
Total	$	6,023	$	3,137

Stockholders’ Deficit (Tables)

12 Months Ended

Dec. 31, 2018

Stockholders Equity Note [Abstract]

Schedule of Reserved Common Stock on an if-Converted Basis for Issuance

As of December 31, 2018 and 2017, the Company had reserved common stock, on an if-converted basis, for issuance as follows:

	December 31, 2018		December 31, 2017
Common stock options issued and outstanding		1,458,080		544,116
Convertible preferred stock		—		30,459,574
Remaining shares available for issuance under 2015 & 2018 Stock Plan		3,801,191		1,855,240
Total		5,259,271		32,858,930

Convertible Preferred Stock (Tables)

12 Months Ended

Dec. 31, 2018

Temporary Equity Disclosure [Abstract]

Schedule of Outstanding Convertible Preferred Stock

As of December 31, 2017, the outstanding convertible preferred stock was as follows (in thousands, except share amounts):

	Shares Authorized		Shares Issued and Outstanding		Liquidation Value		Carrying Value
Series A		49,725,000		12,556,791	$	49,725	$	49,637
Series B		34,653,462		8,750,852		70,000		69,817
Series C		36,580,405		9,151,931		107,000		106,742
Total		120,958,867		30,459,574	$	226,725	$	226,196

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Option Activity Includes Options Granted

The following table, which includes options granted under the 2015 Plan, 2018 Plan and the Non-Plan Option, summarizes option activity:

	Shares Available for Grant			Shares Subject to Outstanding Options			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (in years)
Balance at December 31, 2015		871,226			20,200		$	0.40		8.79
Options authorized		1,133,987			—
Options granted		(746,825	)		746,825		$	0.59
Options exercised		—			(637,107	)	$	0.48
Options repurchased		24,987			—		$	0.40
Balance at December 31, 2016		1,283,375			129,918		$	1.20		9.84
`Options authorized		1,568,397			—
Options granted		(1,063,019	)		1,063,019		$	1.61
Options exercised		—			(643,024	)	$	1.45
Options forfeited or canceled		5,797			(5,797	)	$	1.28
Options repurchased		60,690			—		$	0.49
Balance at December 31, 2017		1,855,240			544,116		$	1.71		9.28
Options authorized		3,570,000
Options granted		(1,645,489	)		1,645,489		$	8.19
Options exercised		—			(720,756	)	$	4.64
Options forfeited or canceled		10,769			(10,769	)	$	5.25
Options repurchased		10,671
Balance at December 31, 2018		3,801,191			1,458,080		$	7.55		8.99
Options outstanding and exercisable as of December 31, 2018					238,721		$	6.49		8.87
Options vested and expected to vest as of December 31, 2018					1,458,080		$	7.55		8.99

Summary of Employee and Non-Employee Stock-based Compensation Expense

The following table summarizes employee and non-employee stock-based compensation expense for the years ended December 31, 2018, 2017 and 2016, and also the allocation within the consolidated statements of operations and comprehensive loss (in thousands):

	Year Ended December 31, 2018		Year Ended December 31, 2017		Year Ended December 31, 2016
Research and development	$	2,255	$	222	$	67
General and administrative		1,619		273		23
Total stock-based compensation	$	3,874	$	495	$	90

Schedule of Assumptions Used to Calculate Fair Value of Stock-Based Compensation

The following assumptions were used to calculate the fair value of stock-based compensation as of December 31, 2018, 2017 and 2016:

	Stock Options			ESPP
	Year Ended December 31, 2018	Year Ended December 31, 2017	Year Ended December 31, 2016	Year Ended December 31, 2018
Risk-free interest rate	1.2% - 3.1%	1.66% - 2.20%	1.2% - 2.45%	2.11% - 2.63%
Expected term (in years)	5.16-9.95	5.95-9.99	6.25-9.84	.5-2.0
Volatility	58.7%-75.5%	67.0%-71.7%	67.0 - 77.8%	54.3% - 65.5%
Dividend yield	0%	0%	0%	0%

Summary of Non-Vested Restricted Stock

A summary of the Company’s non-vested restricted stock for the periods presented is as follows:

	Number of Shares		Weighted Average Grant Date Fair Value		Remaining Contractual Term (Years)
Balance, December 31, 2015		2,372,684	$	0.000396		3.4
Vested during the year		694,444	$	0.000396
Balance, December 31, 2016		1,678,240	$	0.000396		2.4
Vested during the year		694,444	$	0.000396
Balance, December 31, 2017		983,796	$	0.000396		1.4
Vested during the year		694,444	$	0.000396
Balance, December 31, 2018		289,352	$	0.000396		0.4

Net Loss per Share (Tables)

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Computation of Basic and Diluted Net Loss Per Share

The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per share data):

	Year Ended December 31, 2018			Year Ended December 31, 2017			Year Ended December 31, 2016
Numerator:
Net loss	$	(49,594	)	$	(53,082	)	$	(17,970	)
Denominator:
Weighted-average common shares outstanding		36,357,336			3,885,508			3,374,609
Less: weighted-average common shares subject to repurchase		(1,739,099	)		(2,057,246	)		(2,510,626	)
Weighted-average common shares used to compute basic and diluted net loss per share		34,618,237			1,828,262			863,983
Net loss per share, basic and diluted	$	(1.43	)	$	(29.03	)	$	(20.80	)

Summary of Outstanding Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss per Share

The following outstanding potentially dilutive securities were excluded from the computation of diluted net loss per share for the periods presented because including them would have been antidilutive:

	December 31, 2018		December 31, 2017		December 31, 2016
Convertible preferred stock		—		30,459,579		21,307,643
Common stock options issued and outstanding		1,458,079		544,116		129,918
Unvested restricted common stock		289,352		983,796		1,678,240
Unvested early exercised common stock options		927,123		812,769		525,797
Total		2,674,554		32,800,260		23,641,598

Provision for Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Schedule of Provision for Income Taxes Differs from Amount Expected by Applying Federal Statutory Rate to Loss Before Taxes

The provision for income taxes differs from the amount expected by applying the federal statutory rate to the loss before taxes as follows:

	Year Ended December 31, 2018			Year Ended December 31, 2017			Year Ended December 31, 2016
Federal statutory income tax rate		21.00	%		34.00	%		34.00	%
Non-deductible expenses and other		(1.54	)%		(1.23	)%		(0.22	)%
Change in valuation allowance		(19.46	)%		(16.46	)%		(33.78	)%
Remeasurement of federal tax rate change		—			(16.32	)%		—
Total		0.00	%		0.00	%		0.00	%

Schedule of Components of Deferred Tax Assets

As of December 31, 2018 and 2017, the components of the Company’s deferred tax assets are as follows (in thousands):

	Year Ended December 31, 2018			Year Ended December 31, 2017
Deferred tax assets:
Federal and state net operating loss carryforwards	$	16,076		$	11,101
Research and development credits carryforwards		5,087			2,706
Depreciation		3,942			3,489
Deferred Revenue		3,909			—
Other		1,724			1,292
Total deferred tax assets		30,738			18,588
Less valuation allowance		(30,738	)		(18,588	)
Net deferred tax assets	$	—		$	—

Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits

A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands):

	Year Ended December 31, 2018		Year Ended December 31, 2017		Year Ended December 31, 2016
Beginning balance	$	622	$	242	$	19
Additions for tax positions taken in a prior year		8		29		—
Additions for tax positions taken in current year		454		351		223
Ending balance	$	1,084	$	622	$	242

Commitments (Tables)

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Summary of Future Minimum Lease Payments under Non-cancelable Operating Leases

Future minimum lease payments under non-cancelable operating leases as of December 31, 2018 are as follows (in thousands):

Year ending December 31:	Operating Leases
2019	$	2,041
2020		2,105
2021		2,195
2022		2,265
2023		2,339
2024 and beyond		4,487
Total	$	15,432

Selected Unaudited Quarterly Financial Data (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Summary of Unaudited Quarterly Financial Data

The following table summarizes the Company’s unaudited quarterly financial data for the last two years (in thousands, except per share data):

	First Quarter			Second Quarter			Third Quarter			Fourth Quarter
2018
Total revenues	$	1,250		$	1,250		$	4,291		$	1,562
Total operating expenses	$	14,581		$	17,149		$	16,436		$	15,046
Net loss	$	(12,954	)	$	(13,533	)	$	(10,812	)	$	(12,295	)
Net loss per share — basic and diluted	$	(1.37	)	$	(0.32	)	$	(0.25	)	$	(0.28	)
Weighted average number of shares, basic and diluted		9,488,352			42,533,641			42,838,098			43,163,412
2017
Total revenues	$	—		$	—		$	163		$	1,250
Total operating expenses	$	7,300		$	9,664		$	23,297		$	14,594
Net loss	$	(7,200	)	$	(9,550	)	$	(23,122	)	$	(13,210	)
Net loss per share — basic and diluted	$	(4.96	)	$	(5.64	)	$	(11.86	)	$	(5.98	)
Weighted average number of shares, basic and diluted		1,452,215			1,693,150			1,949,258			2,208,065

Organization - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Mar. 21, 2018	Dec. 31, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Common stock, shares authorized	400,000,000	400,000,000	153,993,227
Preferred stock, shares authorized	10,000,000	10,000,000	0
Common stock, par value	$ 0.0001	$ 0.0001	$ 0.0001
Preferred stock, par value	$ 0.0001	$ 0.0001	$ 0.0001
Common Stock
Class Of Stock [Line Items]
Common stock issued		9,200,000
IPO
Class Of Stock [Line Items]
Common stock issued	9,200,000
Common stock offering price	$ 15.00
Net proceeds from issuance of common stock excluding underwriting costs and other offering related costs	$ 124.7
IPO \| Common Stock
Class Of Stock [Line Items]
Convertible preferred stock converted into common stock	30,459,574
Overallotment
Class Of Stock [Line Items]
Common stock issued	1,200,000

Summary of Significant Accounting Policies - Additional Information (Detail)		1 Months Ended	12 Months Ended
	Mar. 09, 2018	Sep. 30, 2017 USD ($)	Dec. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)
Significant Accounting Policies [Line Items]
Reverse stock split			the Company effected a reverse split of all shares of its common and preferred stock at a ratio of 1-for-3.96
Reverse stock split ratio	0.2525
Number of operating segments \| Segment			1
Number of reportable segments \| Segment			1
Deferred offering costs				$ 1,300,000	$ 3,600,000
Taiho Pharmaceutical Co Ltd \| Taiho Agreement
Significant Accounting Policies [Line Items]
Non-refundable, non-creditable upfront cash payments			$ 35,000,000
Payment for option exercise			$ 3,000,000
Minimum
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives			1 year
Impairment charges			$ 0	$ 0
Minimum \| Taiho Pharmaceutical Co Ltd \| Taiho Agreement
Significant Accounting Policies [Line Items]
Payment for option exercise		$ 3,000,000
Maximum
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives			5 years
Maximum \| Taiho Pharmaceutical Co Ltd \| Taiho Agreement
Significant Accounting Policies [Line Items]
Payment for option exercise		$ 15,000,000

Fair Value Measurements - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair value assets transferred from level 1 to level 2	$ 0	$ 0
Fair value assets transferred from level 2 to level 1	0	0
Fair value liabilities transferred from level 1 to level 2	0	0
Fair value liabilities transferred from level 2 to level 1	0	0
Fair value assets transferred into level 3	0	0
Fair value assets transferred out of level 3	0	0
Fair value liabilities transferred into level 3	0	0
Fair value liabilities transferred out of level 3	0	0
Realized gains (loss) on sale or maturity of available-for-sale marketable securities	0	0
Reclassification out of accumulated other comprehensive loss	$ 0	$ 0

Fair Value Measurements - Schedule of Fair Value, Assets Measured on Recurring Basis (Details) - Fair Value On Recurring Basis - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	$ 259,725	$ 175,703
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	45,017	66,478
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	214,708	109,225
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	45,017	66,478
Money Market Funds \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	45,017	66,478
U.S. Government Agency Obligations
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	103,940	57,153
U.S. Government Agency Obligations \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	103,940	57,153
Corporate Securities and Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	110,768	52,072
Corporate Securities and Commercial Paper \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	$ 110,768	$ 52,072

Fair Value Measurements - Schedule of Investments Classified as Available for Sale Securities with Contractual Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Cash and cash equivalents	$ 71,064	$ 98,426
Short-term investments (due within one year)	185,480	77,277
Long-term investments (due between one and two years)	3,181
Total cash and investments in marketable securities	$ 259,725	$ 175,703

Fair Value Measurements - Schedule of Available for Sale Securities at Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 259,833	$ 175,745
Unrealized Loss	(108)	(42)
Fair Value	259,725	175,703
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	45,017	66,478
Fair Value	45,017	66,478
U.S. government agency obligations
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	103,957	57,183
Unrealized Loss	(17)	(30)
Fair Value	103,940	57,153
Corporate Securities and Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	110,859	52,084
Unrealized Loss	(91)	(12)
Fair Value	$ 110,768	$ 52,072

Consolidated Balance Sheet Components - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Total	$ 18,750	$ 15,208
Less: Accumulated depreciation and amortization	(7,643)	(3,978)
Property and equipment, net	11,107	11,230
Scientific Equipment
Property Plant And Equipment [Line Items]
Total	6,628	5,053
Furniture and Equipment
Property Plant And Equipment [Line Items]
Total	813	625
Capitalized Software
Property Plant And Equipment [Line Items]
Total	146	131
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total	10,828	9,280
Construction in Progress
Property Plant And Equipment [Line Items]
Total	$ 335	$ 119

Consolidated Balance Sheet Components - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Depreciation and amortization	$ 3,664	$ 2,612	$ 1,314

Consolidated Balance Sheet Components - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued Liabilities Current [Abstract]
Accrued personnel expenses	$ 2,833	$ 1,026
Accrued research and development expenses	2,816	1,193
Professional fees	211	706
Other	163	212
Total	$ 6,023	$ 3,137

Equity Investment - Additional Information (Details) - USD ($) shares in Millions	1 Months Ended		12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Loss from equity method investments			$ 886,000	$ 416,000	$ 0
Gain on deemed sale from equity method investee			1,229,000	0	$ 0
PACT Pharma
Schedule of Equity Method Investments [Line Items]
Loss from equity method investments			$ 900,000	400,000
Equity method investment agreement termination date			May 31, 2018
Gain on deemed sale from equity method investee			$ 1,200,000
Equity method investments impairment			0	0
PACT Pharma \| Variable Interest Entity
Schedule of Equity Method Investments [Line Items]
Receivable from PACT Pharma			$ 100,000	$ 25,000
PACT Pharma \| Series A Preferred Stock
Schedule of Equity Method Investments [Line Items]
Purchase of common stock, shares	1.0
Purchase of common stock, value	$ 1,000,000
PACT Pharma \| Common Stock
Schedule of Equity Method Investments [Line Items]
Purchase of common stock, shares		3.6

License and Collaboration Agreements - Additional Information (Details)	1 Months Ended					12 Months Ended
	Oct. 31, 2019 USD ($)	Oct. 31, 2018 USD ($)	Jul. 31, 2018 USD ($)	Sep. 30, 2017 USD ($) Program	Aug. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
License And Collaboration Agreements [Line Items]
Deferred revenue, current						$ 6,250,000	$ 5,000,000
Deferred revenue, noncurrent						16,984,000	18,587,000
Taiho Pharmaceutical Co Ltd \| Taiho Agreement
License And Collaboration Agreements [Line Items]
Option period				5 years
Non refundable and non creditable cash payments				$ 35,000,000
Payment received for license agreement							25,000,000
Additional payment to be received for license agreement		$ 5,000,000
Payment for option exercise						$ 3,000,000
Range of royalties receivable on net sales						high single-digits to mid-teens
Royalties payable description						Royalties will be payable on a licensed product-by-licensed product and country-by-country basis during the period of time commencing on the first commercial sale of a licensed product in a country and ending upon the later of: (a) ten (10) years from the date of first commercial sale of such licensed product in such country; and (b) expiration of the last-to-expire valid claim of the Company’s patents covering the manufacture, use or sale or exploitation of such licensed product in such country (the Royalty Term)
Non refundable and non creditable cash payments received						$ 5,000,000	$ 25,000,000
Estimated performance period						4 years	5 years
Remaining non refundable and non creditable cash payments receivable						$ 5,000,000
Payment for first option exercise			$ 3,000,000
Clinical and regulatory milestones achieved						0
Revenue recognized						8,300,000	$ 1,400,000
Deferred revenue, current						6,300,000	5,000,000
Deferred revenue, noncurrent						17,000,000	18,600,000
Taiho Pharmaceutical Co Ltd \| Taiho Agreement \| Minimum
License And Collaboration Agreements [Line Items]
Number of programs, IND enabling studies not initiated \| Program				5
Payment for option exercise				$ 3,000,000
Taiho Pharmaceutical Co Ltd \| Taiho Agreement \| Maximum
License And Collaboration Agreements [Line Items]
Extended option agreement period				7 years
Payment for option exercise				$ 15,000,000
Additional clinical and regulatory milestone payments receivable				130,000,000
Contingent payments receivable				$ 145,000,000
Taiho Pharmaceutical Co Ltd \| Taiho Agreement \| Scenario Forecast
License And Collaboration Agreements [Line Items]
Additional payment to be received for license agreement	$ 5,000,000
WuXi Biologics License Agreement
License And Collaboration Agreements [Line Items]
Upfront and milestone payments							18,500,000
Milestone payments						$ 0
Range of tiered royalty payments on net sales						high single-digits to low teens
WuXi Biologics License Agreement \| Maximum
License And Collaboration Agreements [Line Items]
Clinical, regulatory and commercialization milestone payments					$ 375,000,000
Abmuno License Agreement
License And Collaboration Agreements [Line Items]
Upfront and milestone payments						$ 2,800,000	$ 3,800,000
Abmuno License Agreement \| Maximum
License And Collaboration Agreements [Line Items]
Clinical, regulatory and commercialization milestone payments						$ 101,000,000

Stockholders' Equity (Deficit) - Additional information (Details) - $ / shares	Dec. 31, 2018	Mar. 21, 2018	Dec. 31, 2017
Stockholders Equity Note [Abstract]
Capital stock shares authorized	410,000,000
Common stock, shares authorized	400,000,000	400,000,000	153,993,227
Preferred stock, shares authorized	10,000,000	10,000,000	0
Common stock, par value	$ 0.0001	$ 0.0001	$ 0.0001
Preferred stock, par value	$ 0.0001	$ 0.0001	$ 0.0001

Stockholders' Equity (Deficit) - Schedule of Reserved Common Stock on an if-Converted Basis for Issuance (Details) - shares	Dec. 31, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	5,259,271	32,858,930
Convertible Preferred Stock
Class Of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance		30,459,574
Common Stock Options Issued and Outstanding
Class Of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	1,458,080	544,116
Remaining shares available for issuance under 2015 stock plan
Class Of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	3,801,191	1,855,240

Convertible Preferred Stock - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Mar. 21, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Temporary Equity [Line Items]
Convertible preferred stock, shares outstanding	0		30,459,574
Dividends declared	$ 0
Common stock voting rights	one vote for each share
Common Stock \| IPO
Temporary Equity [Line Items]
Convertible preferred stock converted into common stock		30,459,574
Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, shares outstanding	0		30,459,574	21,307,643	12,556,791
Series A Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, shares outstanding			12,556,791
Dividends declared per share rate	$ 0.24
Stated liquidation preference per share	3.96
Series B Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, shares outstanding			8,750,852
Dividends declared per share rate	0.48
Stated liquidation preference per share	8.00
Series C Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, shares outstanding			9,151,931
Dividends declared per share rate	0.71
Stated liquidation preference per share	$ 11.6915

Convertible Preferred Stock - Schedule of Outstanding Convertible Preferred Stock (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Shares Authorized	0	120,958,867
Shares Issued	0	30,459,574
Shares Outstanding	0	30,459,574
Liquidation Value		$ 226,725,000
Carrying Value	$ 0	$ 226,196,000
Series A
Temporary Equity [Line Items]
Shares Authorized		49,725,000
Shares Issued		12,556,791
Shares Outstanding		12,556,791
Liquidation Value		$ 49,725,000
Carrying Value		$ 49,637,000
Series B
Temporary Equity [Line Items]
Shares Authorized		34,653,462
Shares Issued		8,750,852
Shares Outstanding		8,750,852
Liquidation Value		$ 70,000,000
Carrying Value		$ 69,817,000
Series C
Temporary Equity [Line Items]
Shares Authorized		36,580,405
Shares Issued		9,151,931
Shares Outstanding		9,151,931
Liquidation Value		$ 107,000,000
Carrying Value		$ 106,742,000

Stock-Based Compensation - Additional Information (Details) - USD ($)		3 Months Ended	12 Months Ended
	Jan. 01, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares of common stock reserved for issuance			3,801,191	1,855,240	1,283,375	871,226
Total shares of authorized common stock reserved for future issuance			5,259,271	32,858,930
Common stock, shares outstanding			44,537,946	4,090,898
Description of expected term simplified method			The Company has opted to use the “simplified method” for estimating the expected term of options, whereby the expected term equals the arithmetic average of the vesting term and the original contractual term of the option (generally 10 years).
Expected term			10 years
Expected dividend yield			0.00%
Unrecognized employee compensation costs			$ 11,000,000	$ 1,900,000	$ 400,000
Intrinsic value of shares exercised			600,000	2,500,000	1,200,000
Fair value of shares vested			$ 3,000,000	$ 500,000	$ 100,000
Share-based compensation, vested stock options			14,918	63,878	37,311
Non-Employee Stock-Based Compensation
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares not vested and subject to repurchase			31,388	77,466	47,971
Stock based compensation expense			$ 300,000
Restricted Stock Agreements
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period						4 years
Estimated fair value, price per share						$ 0.0004
Vesting interval period						monthly
Restricted Stock Agreements \| Chief Executive Officer and President
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued						2,777,776
Price per share						$ 0.0004
2018 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares issued or transferred		3,570,000
Number of shares of common stock reserved for issuance			3,801,191
2018 Equity Incentive Plan \| Maximum \| Subsequent Event
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional number of shares reserved for issuance each year	3,570,000
Percentage of common stock outstanding	4.00%
2015 Stock Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares of common stock reserved for issuance		709,558
Shares not vested and subject to repurchase			927,123	812,769
Cash received from early exercise of unvested options			$ 2,800,000	$ 900,000
Award vesting period			48 months
2018 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Purchase date of fair market value trading days		24 months
Total shares of authorized common stock reserved for future issuance		714,000
2018 Employee Stock Purchase Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Purchase price of common stock as percentage of market value		85.00%
Number of shares of common stock that may be purchased by an employee on any purchase date		3,000
Number of shares that may be purchased in any calendar year		$ 25,000
2018 Employee Stock Purchase Plan \| Maximum \| Subsequent Event
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of common stock shares outstanding	1.00%
Common stock, shares outstanding	1,071,000

Stock-Based Compensation - Summary of Options Granted (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shares Available for Grant
Begining balance	1,855,240	1,283,375	871,226
Options authorized	3,570,000	1,568,397	1,133,987
Options granted	(1,645,489)	(1,063,019)	(746,825)
Options forfeited or canceled	10,769	5,797
Options repurchased	10,671	60,690	24,987
Ending balance	3,801,191	1,855,240	1,283,375	871,226
Shares Subject to Outstanding Options
Begining balance	544,116	129,918	20,200
Options granted	1,645,489	1,063,019	746,825
Options exercised	(720,756)	(643,024)	(637,107)
Options forfeited or canceled	(10,769)	(5,797)
Ending balance	1,458,080	544,116	129,918	20,200
Options outstanding and exercisable as of December 31, 2018	238,721
Options vested and expected to vest as of December 31, 2018	1,458,080
Weighted Average Exercise Price Per Share
Begining balance	$ 1.71	$ 1.20	$ 0.40
Options granted	8.19	1.61	0.59
Options exercised	4.64	1.45	0.48
Options forfeited or canceled	5.25	1.28
Options repurchased		0.49	0.40
Ending balance	7.55	$ 1.71	$ 1.20	$ 0.40
Options outstanding and exercisable as of December 31, 2018	6.49
Options vested and expected to vest as of December 31, 2018	$ 7.55
Weighted Average Remaining Contractual Term (in years)	8 years 11 months 26 days	9 years 3 months 10 days	9 years 10 months 2 days	8 years 9 months 14 days
Options outstanding and exercisable, Weighted Average Remaining Contractual Term (in years)	8 years 10 months 13 days
Options vested and expected to vest, Weighted Average Remaining Contractual Term (in years)	8 years 11 months 26 days

Stock-Based Compensation - Summary of Stock-Based Compensation Expense (Details) - Employee And Non Employee - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 3,874	$ 495	$ 90
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	2,255	222	67
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 1,619	$ 273	$ 23

Stock-Based Compensation - Assumptions used to Calculate Fair Value of Stock-Based Compensation (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Expected term (in years)	10 years
Dividend yield	0.00%
Employee Stock Option
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Risk-free interest rate, minimum	1.20%	1.66%	1.20%
Risk-free interest rate, maximum	3.10%	2.20%	2.45%
Volatility, minimum	58.70%	67.00%	67.00%
Volatility, maximum	75.50%	71.70%	77.80%
Dividend yield	0.00%	0.00%	0.00%
Employee stock purchase plan
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Risk-free interest rate, minimum	2.11%
Risk-free interest rate, maximum	2.63%
Volatility, minimum	54.30%
Volatility, maximum	65.50%
Dividend yield	0.00%
Minimum \| Employee Stock Option
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Expected term (in years)	5 years 1 month 28 days	5 years 11 months 12 days	6 years 3 months
Minimum \| Employee stock purchase plan
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Expected term (in years)	6 months
Maximum \| Employee Stock Option
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Expected term (in years)	9 years 11 months 12 days	9 years 11 months 26 days	9 years 10 months 2 days
Maximum \| Employee stock purchase plan
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Expected term (in years)	2 years

Stock-Based Compensation - Summary of Non-Vested Restricted Stock (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Remaining Contractual Term (Years)	8 years 11 months 26 days	9 years 3 months 10 days	9 years 10 months 2 days	8 years 9 months 14 days
Non-vested Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Beginning balance, Number of Shares	983,796	1,678,240	2,372,684
Vested during the year, Number of Shares	694,444	694,444	694,444
Ending balance, Number of Shares	289,352	983,796	1,678,240	2,372,684
Beginning balance, Weighted Average Grant Date Fair Value	$ 0.000396	$ 0.000396	$ 0.000396
Vested during the year, Weighted Average Grant Date Fair Value	0.000396	0.000396	0.000396
Ending balance, Weighted Average Grant Date Fair Value	$ 0.000396	$ 0.000396	$ 0.000396	$ 0.000396
Remaining Contractual Term (Years)	4 months 24 days	1 year 4 months 24 days	2 years 4 months 24 days	3 years 4 months 24 days

Net Loss per Share - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss	$ (12,295)	$ (10,812)	$ (13,533)	$ (12,954)	$ (13,210)	$ (23,122)	$ (9,550)	$ (7,200)	$ (49,594)	$ (53,082)	$ (17,970)
Denominator:
Weighted-average common shares outstanding									36,357,336	3,885,508	3,374,609
Less: weighted-average common shares subject to repurchase									(1,739,099)	(2,057,246)	(2,510,626)
Weighted-average number of shares used to compute basic and diluted net loss per share	43,163,412	42,838,098	42,533,641	9,488,352	2,208,065	1,949,258	1,693,150	1,452,215	34,618,237	1,828,262	863,983
Net loss per share, basic and diluted									$ (1.43)	$ (29.03)	$ (20.80)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Net Loss per Share - Summary of Outstanding Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss per Share (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share	2,674,554	32,800,260	23,641,598
Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share		30,459,579	21,307,643
Common Stock Options Issued and Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share	1,458,079	544,116	129,918
Unvested Restricted Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share	289,352	983,796	1,678,240
Employee Stock Option
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share	927,123	812,769	525,797

Provision for Income Taxes - Schedule of Provision for Income Taxes Differs from Amount Expected by Applying Federal Statutory Rate to Loss Before Taxes (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	21.00%	34.00%	34.00%
Non-deductible expenses and other	(1.54%)	(1.23%)	(0.22%)
Change in valuation allowance	(19.46%)	(16.46%)	(33.78%)
Remeasurement of federal tax rate change		(16.32%)
Total	0.00%	0.00%	0.00%

Provision for Income Taxes - Schedule of Components of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Federal and state net operating loss carryforwards	$ 16,076	$ 11,101
Research and development credits carryforwards	5,087	2,706
Depreciation	3,942	3,489
Deferred Revenue	3,909
Other	1,724	1,292
Total deferred tax assets	30,738	18,588
Less valuation allowance	$ (30,738)	$ (18,588)

Provision for Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Valuation allowance increased value	$ 12,200,000	$ 10,200,000	$ 7,600,000
Net operating loss carryforwards	70,900,000
Interest or penalties accrued	0	0	0
Liability related to uncertain tax positions	0
Reserve for unrecognized tax benefits	1,084,000	$ 622,000	$ 242,000	$ 19,000
Federal
Income Taxes [Line Items]
Net operating loss carryforwards	$ 47,400,000
Net operating loss carryforwards, expiration year	2035
Net operating loss carryforwards amount with no expiration date	$ 23,600,000
Federal \| Earliest Tax Year \| U.S.
Income Taxes [Line Items]
Open tax year	2015
Federal \| Latest Tax Year \| U.S.
Income Taxes [Line Items]
Open tax year	2018
Federal \| Research
Income Taxes [Line Items]
Research tax credits	$ 3,600,000
Research tax credits, expiration year	2035
State
Income Taxes [Line Items]
Net operating loss carryforwards	$ 15,400,000
Net operating loss carryforwards, expiration year	2035
State \| Earliest Tax Year \| California
Income Taxes [Line Items]
Open tax year	2015
State \| Latest Tax Year \| California
Income Taxes [Line Items]
Open tax year	2018
State \| Research
Income Taxes [Line Items]
Research tax credits	$ 3,100,000
Foreign Tax Authority \| Earliest Tax Year \| Australian Taxation Office
Income Taxes [Line Items]
Open tax year	2017
Foreign Tax Authority \| Latest Tax Year \| Australian Taxation Office
Income Taxes [Line Items]
Open tax year	2018

Provision for Income Taxes - Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation Of Unrecognized Tax Benefits Excluding Amounts Pertaining To Examined Tax Returns Roll Forward
Beginning balance	$ 622	$ 242	$ 19
Additions for tax positions taken in a prior year	8	29
Additions for tax positions taken in current year	454	351	223
Ending balance	$ 1,084	$ 622	$ 242

Commitments - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Commitments And Contingencies Disclosure [Abstract]
Purchase commitments cancellation notice period	30 days
Operating lease expiration year	2025
Rent expense	$ 1,600,000	$ 900,000	$ 1,600,000
Deposits for letters of credit	200,000
Minimum sublease rent incom	600,000
Liabilities for Indemnification rights and agreements	$ 0	$ 0	$ 0

Commitments -Summary of Future Minimum Lease Payments under Non-Cancelable Operating Leases (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2019	$ 2,041
2020	2,105
2021	2,195
2022	2,265
2023	2,339
2024 and beyond	4,487
Total	$ 15,432

Employee Benefit Plan - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Compensation And Retirement Disclosure [Abstract]
Defined contribution plan, percentage of employee compensation allowed to be contributed	100.00%
Defined contribution plan, cost	$ 0	$ 0	$ 0

Selected Unaudited Quarterly Financial Data - Summary of Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 1,562	$ 4,291	$ 1,250	$ 1,250	$ 1,250	$ 163			$ 8,353	$ 1,413
Total operating expenses	15,046	16,436	17,149	14,581	14,594	23,297	$ 9,664	$ 7,300	63,212	54,854	$ 18,182
Net loss	$ (12,295)	$ (10,812)	$ (13,533)	$ (12,954)	$ (13,210)	$ (23,122)	$ (9,550)	$ (7,200)	$ (49,594)	$ (53,082)	$ (17,970)
Net loss per share — basic and diluted	$ (0.28)	$ (0.25)	$ (0.32)	$ (1.37)	$ (5.98)	$ (11.86)	$ (5.64)	$ (4.96)	$ (1.43)	$ (29.03)	$ (20.80)
Weighted-average number of shares used to compute basic and diluted net loss per share	43,163,412	42,838,098	42,533,641	9,488,352	2,208,065	1,949,258	1,693,150	1,452,215	34,618,237	1,828,262	863,983

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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