Unity Biotechnology (UBX) 10-K 6 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Mar. 01, 2019	Jun. 29, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Trading Symbol	UBX
Entity Registrant Name	Unity Biotechnology, Inc.
Entity Central Index Key	1,463,361
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Shell Company	false
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		42,856,993
Entity Public Float			$ 423,352,473

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 15,399	$ 7,298
Contribution receivable		1,382
Short-term marketable securities	155,736	79,212
Prepaid expenses and other current assets	1,830	988
Total current assets	172,965	88,880
Property and equipment, net	6,238	6,958
Long-term marketable securities		5,118
Restricted cash	550	550
Other long-term assets	1,622	518
Total assets	181,375	102,024
Current liabilities:
Accounts payable	4,847	2,378
Accrued compensation	3,791	2,181
Accrued and other current liabilities	4,990	3,338
Settlement liability	2,059
Contingent consideration liability	895
Total current liabilities	16,582	7,897
Deferred rent, net of current portion	2,467	3,166
Contingent consideration liability, net of current portion	1,588
Other non-current liabilities	45	118
Total liabilities	20,682	11,181
Commitments and contingencies (Note 7)
Convertible preferred stock, $0.0001 par value; 10,000,000 and 91,739,149 shares authorized as of December 31, 2018 and 2017, respectively; 0 and 28,159,724 shares issued and outstanding as of December 31, 2018 and 2017, respectively; aggregate liquidation preference of $0 and $190,825 as of December 31, 2018 and 2017, respectively		173,956
Stockholders’ equity (deficit):
Common stock, $0.0001 par value; 300,000,000 and 122,000,000 shares authorized as of December 31, 2018 and 2017, respectively; 42,414,294 and 4,830,389 shares issued and outstanding as of December 31, 2018 and 2017, respectively	4	1
Additional paid-in capital	324,663	4,072
Related party promissory notes for purchase of common stock	(201)	(202)
Employee promissory notes for purchase of common stock	(400)
Accumulated other comprehensive loss	(95)	(104)
Accumulated deficit	(163,278)	(86,880)
Total stockholders’ equity (deficit)	160,693	(83,113)
Total liabilities, convertible preferred stock, and stockholders’ equity (deficit)	$ 181,375	$ 102,024

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - USD ($) $ in Thousands	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	10,000,000	91,739,149
Convertible preferred stock, shares issued	0	28,159,724
Convertible preferred stock, shares outstanding	0	28,159,724
Convertible preferred stock, liquidation preference	$ 0	$ 190,825
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	300,000,000	122,000,000
Common stock, shares issued	42,414,294	4,830,389
Common stock, shares outstanding	42,414,294	4,830,389

Statements of Operations and Co

Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Contribution revenue		$ 1,382
Operating expenses:
Research and development	$ 58,907	37,373	$ 13,707
General and administrative	16,016	9,617	5,137
Change in fair value of contingent consideration	4,542
Total operating expenses	79,465	46,990	18,844
Loss from operations	(79,465)	(45,608)	(18,844)
Loss on extinguishment of promissory notes			(9,377)
Interest income (expense), net	3,312	1,055	(2,183)
Other expense, net	(245)	(103)
Net loss	(76,398)	(44,656)	(30,404)
Other comprehensive loss
Unrealized gain (loss) on marketable securities, net of tax	9	(104)
Comprehensive loss	$ (76,389)	$ (44,760)	$ (30,404)
Net loss per share, basic and diluted	$ (2.70)	$ (13.97)	$ (11.42)
Weighted average number of shares used in computing net loss per share, basic and diluted	28,269,907	3,197,516	2,662,841

Statements of Convertible Prefe

Statements of Convertible Preferred Stock and Stockholders' Deficit - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Series A-2 Convertible Preferred Stock	Series B Convertible Preferred Stock	Series C Convertible Preferred Stock	Series A-1, A-2, B and C convertible preferred Stock	Common Stock	Additional Paid-In Capital	Related Party Promissory Notes for Purchase of Commmon Stock	Employee Promissory Notes for Purchase of Commmon Stock	Accumulated Other Comprehensive Loss	Accumulated Deficit
Beginning balance at Dec. 31, 2015	$ (11,745)						$ 1	$ 123	$ (49)			$ (11,820)
Beginning balance, (in shares) at Dec. 31, 2015		8,713,925
Beginning balance at Dec. 31, 2015		$ 7,579
Beginning balance, (in shares) at Dec. 31, 2015							2,054,204
Issuance of convertible preferred stock per share for cash, net of issuance costs (in shares)			4,671,430	11,235,260
Issuance of convertible preferred stock per share for cash, net of issuance costs			$ 4,092	$ 119,418
Issuance of common stock upon exercise of stock options, net of amount related to early exercised options	38							38
Issuance of common stock upon exercise of stock options, net of amount related to early exercised options (in shares)							1,436,902
Vesting of early exercised options	58							58
Issuance of restricted stock							76,271
Common stock granted to third parties/ Issuance of common stock upon initial public offering	446							446
Common stock granted to third parties/ Issuance of common stock upon initial public offering (in shares)							736,161
Stock-based compensation	224							224
Receipt of promissory note from related party for purchase of common stock	(153)								(153)
Net loss	(30,404)											(30,404)
Ending balance at Dec. 31, 2016	(41,536)						$ 1	889	(202)			(42,224)
Ending balance, (in shares) at Dec. 31, 2016		24,620,615
Ending balance at Dec. 31, 2016		$ 131,089
Ending balance (in shares) at Dec. 31, 2016							4,303,538
Issuance of convertible preferred stock per share for cash, net of issuance costs (in shares)				3,539,109
Issuance of convertible preferred stock per share for cash, net of issuance costs				$ 42,867
Issuance of common stock upon exercise of stock options, net of amount related to early exercised options	8							8
Issuance of common stock upon exercise of stock options, net of amount related to early exercised options (in shares)							43,727
Vesting of early exercised options	97							97
Issuance of restricted stock							625,931
Common stock granted to third parties/ Issuance of common stock upon initial public offering	44							44
Common stock granted to third parties/ Issuance of common stock upon initial public offering (in shares)							12,711
Stock-based compensation	3,034							3,034
Unrealized loss on marketable securities, net of tax	(104)										$ (104)
Repurchase of early exercised shares of common stock							(155,518)
Unrealized gain on available-for-sale marketable securities,net of tax	(104)
Net loss	(44,656)											(44,656)
Ending balance at Dec. 31, 2017	$ (83,113)						$ 1	4,072	(202)		(104)	(86,880)
Ending balance, (in shares) at Dec. 31, 2017	28,159,724	28,159,724	10,498,269	14,774,369
Ending balance at Dec. 31, 2017	$ 173,956	$ 173,956	$ 9,214	$ 162,285
Ending balance (in shares) at Dec. 31, 2017	4,830,389						4,830,389
Issuance of convertible preferred stock per share for cash, net of issuance costs (in shares)					3,913,425
Issuance of convertible preferred stock per share for cash, net of issuance costs					$ 59,881
Conversion of Series A-1, A-2, B and C convertible preferredstock to common stock	$ 233,839						$ 2	233,837
Conversion of Series A-1, A-2, B and C convertible preferred stock to common stock (in Shares)						(32,073,149)
Conversion of Series A-1, A-2, B and C convertible preferred stock to common stock						$ (233,837)
Conversion of Series A-1, A-2, B and C convertible preferred stock to common stock (in Shares)							32,073,149
Issuance of common stock upon exercise of stock options, net of amount related to early exercised options (in shares)	501,329
Vesting of early exercised options	$ 584							584
Common stock granted to third parties/ Issuance of common stock upon initial public offering	75,852						$ 1	75,851
Common stock granted to third parties/ Issuance of common stock upon initial public offering (in shares)							5,000,000
Issuance of common stock upon exercise of warrants and stock options,net of amount related to early exercised options	374							374
Issuance of common stock upon exercise of warrants and stock options,net of amount related to early exercised options (in shares)							510,756
Stock-based compensation	9,441							9,441
Unrealized gain on available-for-sale marketable securities,net of tax	9										9
Receipt of promissory note from related party for purchase of common stock	(390)								(390)
Receipt of promissory note from employee for purchase of common stock	(400)									$ (400)
Repayment of promissory note from related party	895							504	391
Net loss	(76,398)											(76,398)
Ending balance at Dec. 31, 2018	$ 160,693						$ 4	$ 324,663	$ (201)	$ (400)	$ (95)	$ (163,278)
Ending balance, (in shares) at Dec. 31, 2018	0
Ending balance (in shares) at Dec. 31, 2018	42,414,294						42,414,294

Statements of Convertible Pre_2

Statements of Convertible Preferred Stock and Stockholders' Deficit (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Amount of early exercised options	$ 1,212	$ 5	$ 408
Issuance cost	$ 9,149
Series A-2 Convertible Preferred Stock
Issuance price per share			$ 0.876
Issuance cost			$ 1
Series B Convertible Preferred Stock
Issuance price per share		$ 12.125	$ 12.125
Issuance cost		$ 43	$ 214
Series C Convertible Preferred Stock
Issuance price per share	$ 15.3317
Issuance cost	$ 119

Condensed Statements of Cash Fl

Condensed Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net loss	$ (76,398)	$ (44,656)	$ (30,404)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,180	1,304	153
Net accretion and amortization of premium and discounts on marketable securities	(955)	182
Stock-based compensation	9,441	3,034	224
Loss on extinguishment of promissory notes			9,377
Non-cash interest expense			2,223
Loss on disposal of property and equipment	45	15
Common stock granted to third party		44	447
Accretion of tenant improvement allowance	(605)	(605)	(403)
Change in fair value of contingent consideration	4,542
Changes in operating assets and liabilities:
Contribution receivable	1,382	(1,382)
Prepaid expenses and other current assets	(842)	(746)	(229)
Other long-term assets	(604)	23	(41)
Accounts payable	2,228	1,198	198
Accrued compensation	1,610	1,607	504
Accrued liabilities and other current liabilities	1,446	1,258	1,046
Deferred rent, net of current portion	(93)	366	27
Other non-current liabilities			480
Net cash used in operating activities	(56,623)	(38,358)	(16,398)
Investing activities
Purchase of marketable securities	(204,086)	(134,465)
Maturities of marketable securities	133,644	49,849
Purchase of investment in stock	(500)		(500)
Purchase of property and equipment	(1,264)	(1,689)	(2,244)
Net cash used in investing activities	(72,206)	(86,305)	(2,744)
Financing activities
Proceeds from issuance of convertible promissory notes payable			16,887
Proceeds from issuance of convertible preferred stock, net of issuance costs	59,881	42,867	90,956
Proceeds from issuance of common stock upon exercise of stock options, net of repurchases	374	(37)	95
Proceeds from initial public offering, net of issuance costs	79,055
Payment of initial public offering costs	(3,201)
Proceeds from repayment of recourse notes	895
Payments made on capital lease obligations	(74)	(55)
Net cash provided by financing activities	136,930	42,775	107,938
Net increase (decrease) in cash, cash equivalents and restricted cash	8,101	(81,888)	88,796
Cash, cash equivalents and restricted cash at beginning of year	7,848	89,736	940
Cash, cash equivalents and restricted cash at end of year	15,949	7,848	89,736
Supplemental Disclosures of Non-Cash Investing and Financing Activities
Conversion and settlement of convertible notes and accrued interest into convertible preferred stock			15,667
Property and equipment included in accounts payable	241	314	98
Property and equipment acquired under capital leases		243
Lessor funded lease incentives included in property and equipment		$ 3,881
Receipt of promissory note for purchase of common stock	400
Receipt of promissory note from related party for purchase of common stock	$ 390		$ 153

Organization

Organization	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	1. Organization Description of Business Unity Biotechnology, Inc. (the “Company”) is a biotechnology company engaged in the research and development of therapeutics to extend human healthspan. The Company devotes substantially all of its time and efforts to performing research and development, raising capital and recruiting personnel. The Company is located in Brisbane, California and was incorporated in the State of Delaware in 2009. Need for Additional Capital The accompanying financial statements have been prepared assuming that the Company will continue as a going concern. Since inception, the Company has incurred net losses and negative cash flows from operations. During the year ended December 31, 2018, the Company incurred a net loss of $76.4 million and used $56.6 million of cash in operations. At December 31, 2018, the Company had an accumulated deficit of $163.3 million and does not expect positive cash flows from operations in the foreseeable future. The Company has historically financed its operations primarily through the issuance and sale of convertible preferred stock and convertible promissory notes. To date, none of the Company’s drug candidates have been approved for sale and therefore the Company has not generated any revenue from contracts with customers. The Company has evaluated and concluded there are no conditions or events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern for a period of one year following the date that these financial statements are issued. Management expects operating losses to continue for the foreseeable future. As a result, the Company will need to raise additional capital. If sufficient funds on acceptable terms are not available when needed, the Company could be required to significantly reduce its operating expenses and delay, reduce the scope of, or eliminate one or more of its development programs. Failure to manage discretionary spending or raise additional financing, as needed, may adversely impact the Company’s ability to achieve its intended business objectives.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation These financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). Reverse Stock Split On April 19, 2018, the Company’s board of directors approved an amendment to the Company’s amended and restated certificate of incorporation to effect a 1-for-2.95 reverse split (“Reverse Split”) of shares of the Company’s common and convertible preferred stock, which was effected on April 20, 2018. The par value and authorized shares of common stock and convertible preferred stock were not adjusted as a result of the Reverse Split. All of the share and per share information included in the accompanying financial statements has been adjusted to reflect the Reverse Split. Initial Public Offering On May 7, 2018, the Company closed its initial public offering (“IPO”), of 5,000,000 shares of common stock, at an offering price to the public of $17.00 per share. The Company received net proceeds of approximately $75.9 million, after deducting underwriting discounts, commissions and offering related transaction costs of approximately $9.1 million. In connection with the IPO, all of the Company’s outstanding shares of convertible preferred stock were automatically converted into 32,073,149 shares of common stock. In addition, all of the Company’s convertible preferred stock warrants were converted into warrants to purchase shares of common stock. In connection with the completion of its IPO, on May 7, 2018, the Company’s certificate of incorporation was amended and restated to provide for 300,000,000 authorized shares of common stock with a par value of $0.0001 per share and 10,000,000 authorized shares of preferred stock with a par value of $0.0001 per share. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. The Company bases its estimates on historical experience and market-specific or other relevant assumptions that it believes are reasonable under the circumstances. The amounts of assets and liabilities reported in the Company’s balance sheets and the amount of expenses and income reported for each of the periods presented are affected by estimates and assumptions, which are used for, but are not limited to, determining the fair value of assets and liabilities, contingent consideration liability, common stock valuation, and stock-based compensation. Actual results could differ from such estimates or assumptions. Segments The Company has one operating segment. The Company’s chief operating decision maker, its Chief Executive Officer, manages the Company’s operations on a consolidated basis for the purposes of allocating resources. Cash, Cash Equivalents and Restricted Cash The Company considers all highly liquid investments with original maturities of 90 days or less from the date of purchase to be cash equivalents. Cash equivalents primarily include money market funds that invest in U.S. Treasury obligations which are stated at fair value. The Company has issued a letter of credit under a lease agreement which has been collateralized. This cash is classified as noncurrent restricted cash on the balance sheet based on the term of the underlying lease. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets that sum to the total of the same amounts shown in the statements of cash flows. December 31, 2018 2017 2016 (in thousands) Cash and cash equivalents $ 15,399 $ 7,298 $ 89,286 Restricted cash 550 550 450 Total cash, cash equivalents, and restricted cash $ 15,949 $ 7,848 $ 89,736 Marketable Securities The Company generally invests its excess cash in investment grade, short to intermediate-term, fixed income securities. Such investments are considered available-for-sale, and reported at fair value with unrealized gains and losses included as a component of stockholders’ deficit. Marketable securities with original maturities of greater than 90 days from the date of purchase but less than one year from the balance sheet date are classified as short-term, while marketable securities with maturities in one year or beyond one year from the balance sheet date are classified as long-term. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity, which is included in interest income on the statements of operations and comprehensive loss. Realized gains and losses and declines in value judged to be other-than-temporary, if any, on marketable securities are included in interest income (expense), net. The cost of securities sold is determined using the specific identification method. The Company periodically evaluates whether declines in fair values of its marketable securities below their book value are other-than-temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the marketable security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any marketable securities before recovery of its amortized cost basis. Factors considered include quoted market prices, recent financial results and operating trends, implied values from any recent transactions or offers of investee securities, credit quality of debt instrument issuers, other publicly available information that may affect the value of the marketable security, duration and severity of the decline in value, and management’s strategy and intentions for holding the marketable security. To date, the Company has not recorded any impairment charges on its marketable securities related to other-than-temporary declines in market value. Fair Value of Financial Instruments The Company’s financial instruments during the periods presented consist of cash and cash equivalents, restricted cash, contribution receivable, marketable securities, prepaid expenses and other current assets, accounts payable, accrued compensation, accrued and other current liabilities. Fair value estimates of these instruments are made at a specific point in time, based on relevant market information. These estimates may be subjective in nature and involve uncertainties and matters of significant judgment. Concentrations of Risk Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash, cash equivalents, restricted cash, marketable securities and contribution receivable. Substantially all of the Company’s cash and cash equivalents and restricted cash is deposited in accounts with financial institutions that management believes are of high credit quality. Such deposits have and will continue to exceed federally insured limits. The Company maintains its cash with accredited financial institutions and accordingly, such funds are subject to minimal credit risk. The Company has not experienced any losses on its cash deposits. The contribution receivable is unsecured and is concentrated with one third-party organization, and accordingly the Company may be exposed to credit risk. To date, the Company has not experienced any loss related to its contributions receivable. The Company’s investment policy limits investments to certain types of securities issued by the U.S. government, its agencies and institutions with investment-grade credit ratings and places restrictions on maturities and concentration by type and issuer. The Company is exposed to credit risk in the event of a default by the financial institutions holding its cash, cash equivalents, restricted cash and marketable securities and issuers of marketable securities to the extent recorded on the balance sheets. As of December 31, 2018, the Company had no off-balance sheet concentrations of credit risk. The Company depends on third-party suppliers for key raw materials used in its manufacturing processes and is subject to certain risks related to the loss of these third-party suppliers or their inability to supply the Company with adequate raw materials. Contribution Revenue and Receivables The Company recognizes contribution revenue related to the receipt of cash from third-party resource providers not considered to be customers and where the transfer of assets is not an exchange transaction or financing of research and development. Contribution revenue and related receivables are recognized for conditional contributions as the conditions related to the contribution are relieved. In July 2017, the Company entered an arrangement with a third-party organization under which the Company would be provided with up to $1.5 million of funding for the performance of certain research and development activities during the 90-day period following the arrangement in pursuit of the third-party organization’s philanthropic mission. All conditions related to this contribution were met during 2017 and the Company recognized $1.4 million under this arrangement, which was recorded as contribution revenue in the statement of operations and a contribution receivable on the balance sheet. Research and Development Expenses Costs related to research, design and development of drug candidates are charged to research and development expense as incurred. Research and development costs include, but are not limited to, payroll and personnel expenses for personnel contributing to research and development activities, laboratory supplies, outside services, licenses acquired to be used in research and development and allocated overhead, including rent, equipment, depreciation and utilities. Payments made prior to the receipt of goods or services to be used in research and development are deferred and recognized as expense in the period in which the related goods are received or services are rendered. Such payments are evaluated for current or long-term classification based on when they will be realized. Contingent Consideration Liability The Company has entered into and may continue to enter into, license agreements to access and utilize certain technology. In each case, the Company evaluates if the license agreement results in the acquisition of an asset or a business. To date none of the Company’s license agreements have been considered an acquisition of a business. For asset acquisitions, the upfront payments to acquire such licenses, as well as any future milestone payments made before product approval, are immediately recognized as research and development expense when due, provided there is no alternative future use of the rights in other research and development projects. These license agreements also include contingent consideration in the form of additional issuances of the Company’s common stock based on the achievement of certain milestones. For asset acquisitions, the Company assesses on a continuous basis whether such contingent consideration meets the definition of a derivative and can or cannot be classified within stockholders’ equity, until such time that equity classification criteria are met or the milestones expire. The derivative related to this contingent consideration is measured at fair value as of each balance sheet date with the related change in fair value being reflected in operating expenses. Upon a reassessment event that results in the contingent consideration no longer meeting the definition of a derivative and/or meeting equity classification critera, the final change in fair value of the instrument is recorded within operating expenses and the liability is reclassified into stockholders’ equity. Variable Interest Entities The Company reviews agreements it enters into with third-party entities, pursuant to which the Company may have a variable interest in the entity, in order to determine if the entity is a variable interest entity (“VIE”). If the entity is a VIE, the Company assesses whether or not it is the primary beneficiary of that entity. In determining whether the Company is the primary beneficiary of an entity, the Company applies a qualitative approach that determines whether it has both (i) the power to direct the economically significant activities of the entity and (ii) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity. If the Company determines it is the primary beneficiary of a VIE, it consolidates that VIE into the Company’s financial statements. The Company’s determination about whether it should consolidate such VIEs is made continuously as changes to existing relationships or future transactions may result in a consolidation or deconsolidation event. Property and Equipment, Net Property and equipment are stated at cost, less accumulated depreciation. Depreciation is calculated using the straight-line method over the estimated useful lives of the respective assets, generally three years. Leasehold improvements are amortized on a straight-line basis over the shorter of their estimated useful lives or the term of the lease. Depreciation and amortization begins at the time the asset is placed in service. Maintenance and repairs are charged to expense as incurred and costs of improvement are capitalized. Impairment of Long-Lived Assets The Company evaluates long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be fully recoverable. If indicators of impairment exist and the undiscounted future cash flows that the assets are expected to generate are less than the carrying value of the assets, the Company reduces the carrying amount of the assets through an impairment charge, to their estimated fair values based on a discounted cash flow approach or, when available and appropriate, to comparable market values. No impairment losses have been recorded for the periods presented. Leases The Company leases office space and laboratory facilities under non-cancelable operating lease agreements and recognizes related rent expense on a straight-line basis over the term of the lease. Incentives granted under the Company’s facilities lease, including allowances to fund leasehold improvements and rent holidays, and are recognized as reductions to rental expense on a straight-line basis over the term of the lease. Lessor funded leasehold improvement incentives not yet received are recorded in prepaid expense and other current assets on the balance sheet. The Company does not assume renewals in its determination of the lease term unless they are deemed to be reasonably assured at the inception of the lease and begins recognizing rent expense on the date that it obtains the legal right to use and control the leased space. Deferred rent consists of the difference between cash payments and the rent expense recognized. The Company entered into capital lease agreements for certain equipment with a lease term of three years. The current portion of capital lease obligations is included in accrued and other liabilities and the noncurrent capital lease obligations is included in other noncurrent liabilities in the balance sheet. Convertible Preferred Stock The Company records all shares of convertible preferred stock at their respective issuance price less issuance costs on the dates of issuance. Upon the occurrence of certain change in control events that are outside the Company’s control, including liquidation, sale or transfer of the Company, holders of the convertible preferred stock can cause redemption for cash. Therefore, convertible preferred stock is classified outside of stockholders’ deficit on the balance sheet as events triggering the liquidation preferences are not solely within the Company’s control. The carrying values of the convertible preferred stock are adjusted to their liquidation preferences when and if it becomes probable that such an event will occur. Stock-Based Compensation The Company measures employee and director stock-based compensation expense for all stock-based awards based on their grant date fair value. For stock-based awards with service conditions only, stock-based compensation expense is recognized over the requisite service period using the straight-line method. For awards with performance conditions, the Company evaluates the probability of achieving performance condition at each reporting date. The Company begins to recognize stock-based compensation expense using an accelerated attribution method when it is deemed probable that the performance condition will be met. Forfeitures are recognized as they occur. The Company uses the Black-Scholes option-pricing model to estimate the fair value of stock option awards that do not contain market conditions. The Black-Scholes option-pricing model requires assumptions to be made related to the expected term of an award, expected dividends, expected volatility and risk-free rate. The Company uses the Monte Carlo simulation models to estimate the fair value of stock option awards that contain market conditions. The Monte Carlo simulation models require the use of subjective and complex assumptions which determine the fair value of such awards including price volatility of the underlying stock and derived service periods. The Company recognizes stock-based compensation expense for stock options granted to non-employees based on the estimated fair value of the award as it is more readily measurable than the fair value of the services received. The fair value of stock options granted to non-employees is estimated at grant date and re-measured at each reporting period using the Black-Scholes option-pricing model until the awards vest and the resulting change in value, if any, is recognized in the statements of operations. Income Taxes The Company uses the asset and liability method of accounting for income taxes, in which deferred tax assets and liabilities are recognized for future tax consequences attributable to the differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using the enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be reversed. The effect on deferred tax assets and liabilities of a change in tax rates is recognized as income in the period that includes the enactment date. A valuation allowance is established if it is more likely than not that all or a portion of the deferred tax asset will not be realized. The Company’s tax positions are subject to income tax audits. The Company recognizes the tax benefit of an uncertain tax position only if it is more likely than not that the position is sustainable upon examination by the taxing authority, based on the technical merits. The tax benefit recognized is measured as the largest amount of benefit which is more likely than not to be realized upon settlement with the taxing authority. The Company recognizes interest accrued and penalties related to unrecognized tax benefits in its tax provision. The Company evaluates uncertain tax positions on a regular basis. The evaluations are based on a number of factors, including changes in facts and circumstances, changes in tax law, correspondence with tax authorities during the course of the audit, and effective settlement of audit issues. The provision for income taxes includes the effects of any accruals that the Company believes are appropriate, as well as the related net interest and penalties. Net Loss per Common Share Basic net loss per share is calculated by dividing net loss by the weighted average number of shares outstanding for the period. Diluted net loss per share is calculated by dividing net loss by the weighted average number of shares of common stock and potential dilutive common stock equivalents outstanding during the period if the effect is dilutive. The calculation of diluted earnings (loss) per share also requires that, to the extent the presumed issuance of additional shares as contingent consideration is dilutive to earnings (loss) per share for the period, adjustments to net income or net loss used in the calculation are required to remove the change in fair value of the contingent consideration liability for the period. Likewise, adjustments to the denominator are required to reflect the related dilutive shares. In all periods presented, the Company’s outstanding stock options, convertible preferred stock, early exercised common stock subject to future vesting, restricted stock accounted for as options common and preferred stock warrants and presumed issuance of additional shares as contingent consideration were excluded from the calculation of diluted net loss per share because their effects were antidilutive. Comprehensive Loss Comprehensive loss includes net loss and certain changes in stockholders’ deficit that are excluded from net loss, primarily unrealized losses on the Company’s marketable securities. Recently Issued Accounting Pronouncements In August 2018, the FASB issued ASU No. 2018-15, Intangibles (Topic 350): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. This new standard also requires customers to expense the capitalized implementation costs of a hosting arrangement that is a service contract over the term of the hosting arrangement. This standard is effective for the Company for annual reporting periods beginning after December 15, 2020, and interim periods within annual periods beginning after December 15, 2021. This new standard can be applied either retrospectively or prospectively to all implementation costs incurred after the date of adoption. The Company is currently evaluating the impact of adoption on its financial statements. In August 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the SEC adopted amendments to certain disclosure requirements in Securities Act Release No. 33-10532, Disclosure Update and Simplification. The Company adopted these SEC amendments on November 5, 2018 and will present the analysis of changes in stockholders’ equity in its interim financial statements in its March 31, 2019 Form 10-Q. The Company does not anticipate that the adoption of these SEC amendments will have a material effect on the Company’s financial position, results of operations, cash flows or shareholders’ equity. In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting In January 2017, the FASB issued ASU No. 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230: Classification of Certain Cash Receipts and Cash Payments) In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities In May 2017, the FASB issued ASU No. 2017-09, Compensation-Stock Compensation (Topic 718)- Scope of Modification Accounting

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The Company determines the fair value of financial and non-financial assets and liabilities based on the assumptions that market participants would use in pricing the asset or liability in an orderly transaction between market participants at the measurement date. The identification of market participant assumptions provides a basis for determining what inputs are to be used for pricing each asset or liability. A fair value hierarchy has been established which gives precedence to fair value measurements calculated using observable inputs over those using unobservable inputs. This hierarchy prioritized the inputs into three broad levels as follows: • Level 1: Quoted prices in active markets for identical instruments • Level 2: Other significant observable inputs (including quoted prices in active markets for similar instruments) • Level 3: Significant unobservable inputs (including assumptions in determining the fair value of certain investments) The carrying amounts of financial instruments such as cash and cash equivalents, restricted cash, contribution receivable, prepaid expenses and other current assets, accounts payable, accrued compensation, accrued and other current liabilities approximate the related fair values due to the short maturities of these instruments. The Company’s financial assets subject to fair value measurements on a recurring basis and the level of inputs used in such measurements were as follows (in thousands): December 31, 2018 Total Level 1 Level 2 Level 3 Assets: Cash equivalents: Money market funds $ 14,131 $ 14,131 $ — $ — Total cash equivalents 14,131 14,131 — — Short-term marketable securities: U.S. treasuries 34,121 — 34,121 — U.S. and foreign commercial paper 10,635 — 10,635 — U.S. and foreign corporate debt securities 26,533 — 26,533 — Asset-backed securities 2,748 — 2,748 — U.S. government debt securities 81,699 — 81,699 — Total short-term marketable securities 155,736 — 155,736 — Total assets subject to fair value measurements on a recurring basis $ 169,867 $ 14,131 $ 155,736 $ — Liabilities: Contingent consideration liability $ 2,483 $ — $ — $ 2,483 Total liabilities subject to fair value measurements on a recurring basis $ 2,483 $ — $ — $ 2,483 December 31, 2017 Total Level 1 Level 2 Level 3 Assets: Cash equivalents: Money market funds $ 5,709 $ 5,709 $ — $ — Total cash equivalents 5,709 5,709 — — Short-term marketable securities: U.S. and foreign commercial paper 6,359 — 6,359 — U.S. and foreign corporate debt securities 16,149 — 16,149 — Asset-backed securities 14,588 — 14,588 — U.S. government debt securities 40,362 — 40,362 — U.S. treasuries 1,754 — 1,754 — Total short-term marketable securities 79,212 — 79,212 — Long-term marketable securities: Asset-backed securities 2,742 — 2,742 — U.S. government debt securities 2,376 — 2,376 — Total long-term marketable securities 5,118 — 5,118 — Total marketable securities 84,330 — 84,330 — Total assets subject to fair value measurements on a recurring basis $ 90,039 $ 5,709 $ 84,330 $ — The Company estimates the fair value of its money market funds, U.S. and foreign commercial paper, U.S. and foreign corporate debt securities, asset-backed securities, U.S. treasuries and U.S. government debt securities by taking into consideration valuations obtained from third-party pricing services. The pricing services utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly, to estimate fair value. These inputs include reported trades of and broker/dealer quotes on the same or similar securities, issuer credit spreads; benchmark securities; prepayment/default projections based on historical data; and other observable inputs. The fair value of the contingent consideration liability includes inputs not observable in the market and thus represents a Level 3 measurement. The Company has recorded a contingent consideration liability related to two agreements executed in February 2016 with a privately held clinical-stage biopharmaceutical company: (a) a license agreement granting the Company the right to research, develop, and seek and obtain marketing approval for an initial licensed compound, and (b) a compound library and option agreement granting the Company the right to identify and take licenses to additional compounds, in each case for the treatment of indications outside of oncology (collectively, the “Commercial Agreements”). The Commercial Agreements include contingent consideration of up to an aggregate of 666,670 additional shares of common stock to be issued based on achievement of certain specified clinical development and sales milestone events. The Company valued the contingent consideration liability using a probability-weighted valuation approach model which reflects the probability and timing of future issuances of shares. The probability of achieving the defined milestones for each licensed product was estimated by the Company’s management. Total contingent consideration may change significantly as preclinical and clinical development under the Commercial Agreements progresses and additional data is obtained, impacting the Company’s assumptions regarding probabilities of successful achievement of related development and commercial milestones used to estimate the fair value of the liability and the timing in which they are expected to be achieved. For example, significant increases in the estimated probability of achieving a milestone would result in a significantly higher fair value measurement while significant decreases in the estimated probability of achieving a milestone would result in a significantly lower fair value measurement. The potential outstanding contingent consideratin value results in shares to be issued ranging from zero, if none of the milestones are achieved, to a maximum of $8.7 million at December 31, 2018 (using the Company’s stock price as of December 31, 2018). As of December 31, 2018 and 2017, none of the commercial milestones had been achieved and no royalties were due from the sales of licensed products. As of December 31, 2018, the Company determined that the net settlement criteria of the definition of a derivative had been met for 133,333 shares of common stock to the third parties. The Company issued 106,666 of these shares in January 2019 and will issue the remaining 26,667 shares in early 2019 and recorded a settlement liability of $2.0 million at December 31, 2018. The Company recorded a contingent consideration liability of $2.5 million at December 31, 2018 related to additional potential shares subject to the achievement of certain specified clinical development and sales milestone events under the agreements. No liability was recorded at December 31, 2017 as the net settlement criteria were not met. The following table provides a reconciliation of assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs (Level 3) (in thousands): Amount Balance at December 31, 2017 $ — Additions — Settlements — Change in fair value 4,542 Balance at December 31, 2018 $ 4,542 There were no transfers within the hierarchy during the years ended December 31, 2018 and 2017. See Note 4 for further information regarding the carrying value of the Company's financial instruments.

Marketable Securities

Marketable Securities	12 Months Ended
	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Marketable Securities	4. Marketable Securities Marketable securities, which are classified as available-for-sale, consisted of the following (in thousands): December 31, 2018 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value Cash equivalents: Money market funds $ 14,131 $ — $ — $ 14,131 Total cash equivalents 14,131 — — 14,131 Short-term marketable securities: U.S. and foreign commercial paper 10,638 — (3 ) 10,635 U.S. and foreign corporate debt securities 26,552 2 (21 ) 26,533 Asset-backed securities 2,750 — (2 ) 2,748 U.S. government debt securities 81,755 1 (57 ) 81,699 U.S. treasuries 34,136 1 (16 ) 34,121 Total short-term marketable securities 155,831 4 (99 ) 155,736 Total marketable securities $ 169,962 $ 4 $ (99 ) $ 169,867 December 31, 2017 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value Cash equivalents: Money market funds $ 5,709 $ — $ — $ 5,709 Total cash equivalents 5,709 — — 5,709 Short-term marketable securities: U.S. and foreign commercial paper 6,369 — (10 ) 6,359 U.S. and foreign corporate debt securities 16,162 — (13 ) 16,149 Asset-backed securities 14,604 — (16 ) 14,588 U.S. government debt securities 40,418 — (56 ) 40,362 U.S. treasuries 1,754 — — 1,754 Total short-term marketable securities 79,307 — (95 ) 79,212 Long-term marketable securities: Asset-backed securities 2,752 — (10 ) 2,742 U.S. government debt securities 2,375 1 — 2,376 Total long-term marketable securities 5,127 1 (10 ) 5,118 Total marketable securities $ 90,143 $ 1 $ (105 ) $ 90,039 There have been no significant realized gains or losses on available-for-sale securities for the periods presented. Available-for-sale debt securities that were in a continuous loss position but were not deemed to be other than temporarily impaired were immaterial at both December 31, 2018 and 2017. The Company does not intend to and believes it is not more likely than not that it will be required to sell these securities before their maturities. See Note 3 for further information regarding the fair value of the Company's financial instruments.

License Agreements

License Agreements	12 Months Ended
	Dec. 31, 2018
Research And Development [Abstract]
License Agreements	5. License Agreements License and Compound Library and Option Agreement In February 2016, the Company entered into a license agreement with a privately held clinical-stage biopharmaceutical company (the “Licensor”) to research, develop, and seek and obtain marketing approval for a licensed compound. In February 2016, in conjunction with this license agreement, the Company also entered into a compound library and option agreement with the Licensor to identify compounds with potential utility in the treatment of age-related conditions other than indications in oncology (collectively, the “Commerical Agreements”). As part of these agreements, the Company issued 533,335 shares of common stock to the Licensor and 133,333 shares of common stock to an academic institution from whom the Licensor had previously licensed the technology. The Commercial Agreements referenced above include cash payments of up to $70.3 million as well as the equity payments of up to an aggregate 666,670 additional shares of common stock, in each case to be issued based on the Company’s achievement of certain specified clinical development and sales milestone events. The milestones include the filing of an investigational drug application, the commencement of clinical studies, Food and Drug Administration and/or European Medicines Agency approval, and a net sales threshold. The license agreement also includes tiered royalties in the low-single digits based on sales of licensed products. In December 2018, the Company elected to advance a second compound into formal preclinical development which gave rise to an obligation under the compound library and option agreement to issue an additional 133,333 shares of common stock to the Licensor and the academic institution. In connection with the issuance of these shares, the Company issued 106,666 shares of common stock to the Licensor in January 2019 and the remaining 26,667 will be issued to the academic institution in 2019. The Company recorded a settlement liability of $ 2.0 2.5 none no In April 2016, in connection with the Commercial Agreements the Company purchased an equity interest in an affiliate of the Licensor for an aggregate purchase price of $0.5 million. The equity interest represents an insignificant level of ownership in the entity and has been recorded within other assets in the Company’s balance sheet. In May 2018 these shares were exchanged for new shares of a newly formed affiliate of the Licensor as part of a reorganization of those entities. The Company also invested an additional $0.5 million in the newly formed affiliate of the Licensor in May 2018. The Company agreed to provide funding to the Licensor for research and development work performed at a cost of up to $2.0 million through February 2020. The research and development expense under the research services agreement was $0.5 million and $0.5 million for the years ended December 31, 2018 and 2017. Under the consolidation guidance, the Company determined that the Licensor is a VIE. The Company does not have the power to direct the activities that most significantly affect the economic performance of this entity and as such the Company is not the primary beneficiary and consolidation is not required. As of December 31, 2018 and 2017, the Company has not provided financial, or other, support to the Licensor that was not contractually required. Other License Agreements with Research Institutions The Company has entered into license agreements with various research institutions which have provided the Company with rights to patents, and in certain cases, research “know-how” and proprietary research tools to research, develop and commercialize drug candidates. In addition to upfront consideration paid to these various research institutions in either cash or shares of the Company’s common stock, the Company may be obligated to pay milestone payments in cash or the issuance of the Company’s common stock upon achievement of certain specified clinical development and/or sales events. The contingent consideration liability considered to be a derivative associated with the potential issuance of common stock related to these license agreements was not significant at December 31, 2018 or 2017. To date, none of these events has occurred and no contingent consideration, milestone or royalty payments have been recognized.

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Balance Sheet Components	6. Balance Sheet Components Property and Equipment, Net Property and equipment, net, consists of the following: December 31, 2018 2017 (in thousands) Laboratory equipment $ 4,162 $ 2,614 Computer equipment 247 137 Furniture and fixtures 113 105 Leasehold improvements 5,366 5,346 Construction in progress — 226 Total property and equipment 9,888 8,428 Less: accumulated depreciation and amortization (3,650 ) (1,470 ) Total property and equipment, net $ 6,238 $ 6,958 Depreciation expense related to property and equipment was $2.2 million, 1.3 million and $0.2 million for the years ended December 31, 2018, 2017 and 2016 respectively. Accrued and Other Current Liabilities Accrued and other current liabilities consist of the following: December 31, 2018 2017 (in thousands) Accrued research and development $ 1,837 $ 2,105 Deferred rent, current portion 783 702 Professional fees 26 70 Liability related to early exercise shares 885 257 Accrued other 1,459 204 $ 4,990 $ 3,338

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies Indemnifications The Company indemnifies each of its officers and directors for certain events or occurrences, subject to certain limits, while the officer or director is or was serving at the Company’s request in such capacity, as permitted under Delaware law and in accordance with the Company’s amended and restated certificate of incorporation and bylaws. The term of the indemnification period lasts as long as an officer or director may be subject to any proceeding arising out of acts or omissions of such officer or director in such capacity. The maximum amount of potential future indemnification is unlimited; however, the Company currently holds director and officer liability insurance. This insurance allows the transfer of risk associated with the Company’s exposure and may enable the Company to recover a portion of any future amounts paid. The Company believes that the fair value of these indemnification obligations is minimal. Accordingly, the Company has not recognized any liabilities relating to these obligations for any period presented. Operating Lease In May 2016, the Company executed a non-cancellable lease agreement for office and laboratory space in Brisbane, California which commenced in May 2016 and continues through October 2022. The lease agreement includes an escalation clause for increased rent and a renewal provision allowing the Company to extend this lease for an additional four years by giving the landlord written notice of the election to exercise the option at least fifteen months prior to the original expiration of the lease term. The lease provides for monthly base rent amounts escalating over the term of the lease and the lessor provided the Company a $3.9 million tenant improvement allowance to complete the laboratory and office renovation which was recorded as deferred rent liability and leasehold improvement within property and equipment, net. In May 2017, the Company entered into an amendment to expand the leased space and received a three-month rent holiday for the expanded space. As of December 31, 2018, the Company’s future minimum payments under the noncancelable operating lease is as follows (in thousands): Amount 2019 $ 2,012 2020 2,072 2021 2,135 2022 1,621 Total future minimum lease payments $ 7,840 Rent expense was $1.8 million, $2.0 million and $0.8 million and for the years ended December 31, 2018, 2017 and 2016, respectively.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2018
Related Party Transactions [Abstract]
Related Party Transactions	8. Related-Party Transactions Recourse Notes In December 2015, April 2016, and July 2016, the Company issued three full-recourse promissory notes to two executive officers for an aggregate principal amount of $0.2 million with an interest rate of 2.5% per annum. All of the principal was used to early exercise options for 667,253 shares of the Company’s common stock, in aggregate. All of these related party full-recourse notes were repaid on April 4, 2018 in accordance with the terms of such notes. In October 2017, the Company issued two promissory notes to an executive officer for $1.6 million and $0.5 million, each with an interest rate of 1.85% per annum. The aggregate principal amount of $2.1 million was used to purchase 625,084 shares of restricted stock. The promissory notes were considered to be non-recourse in substance and accordingly, the shares sold subject to such promissory notes are considered to be an option for accounting purposes. In April 2018, the Company’s board of directors approved the forgiveness of all outstanding principal and accrued interest of the $1.6 million non-recourse promissory note. The non-recourse promissory note outstanding of $0.5 million was repaid on April 4, 2018 in accordance with the terms of the note. The forgiveness of the promissory note was accounted for as a modification of a share-based payment. The Company recorded an incremental charge of $1.5 million related to the modification for the year ended December 31, 2018. In January 2018, the Company issued full-recourse promissory notes to an executive and an executive officer of the Company for an aggregate principal amount of $0.4 million with an interest rate of 2.5% per annum. All of the principal was used to early exercise options for 114,406 shares of the Company’s common stock. The full recourse note of $0.2 million for the executive officer was repaid on April 4, 2018 in accordance with the terms of the note Financing Activities During the year ended December 31, 2018, the Company issued convertible preferred stock for total proceeds of $3.0 million to shareholders who are considered to be related parties. During the year ended December 31, 2017, the Company issued additional shares of Series B convertible preferred stock for total proceeds of $8.0 million to one of these related party shareholders. During the year ended December 31, 2016, the Company issued convertible preferred stock and convertible notes for total proceeds of $32.8 million to shareholders and certain executive officers who are considered to be related parties. All of the convertible notes converted into shares of series B preferred stock during 2016. Other In 2017, the Company entered into a master services agreement with a significant shareholder who was considered a related party. The Company incurred a total of $0.4 million and $0.6 million of research and development expenses during the years ended December 31, 2018 and 2017, respectively, related to this agreement.

Convertible Preferred Stock and

Convertible Preferred Stock and Common Stock	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Convertible Preferred Stock and Common Stock	9. Convertible Preferred Stock and Common Stock Convertible Preferred Stock The Company is authorized to issue two classes of stock: convertible preferred stock and common stock. Convertible preferred stock is carried at the issuance price, net of issuance costs. In July 2013, the Company sold an aggregate of 2,887,086 shares of Series A-1 convertible preferred stock at $0.864 per share for gross proceeds of $2.0 million. From January 2014 through March 2015, the Company closed three tranches of Series A-2 convertible preferred stock financing and sold an aggregate of 5,826,839 shares of Series A-2 convertible preferred stock at $0.876 per share for gross proceeds of $4.9 million. In February 2016, the Company closed the final tranche of Series A-2 convertible preferred stock financing by selling an aggregate of 4,671,430 shares of Series A-2 convertible preferred stock at $0.876 per share for gross proceeds of $4.0 million. In October 2016, the Company closed the first tranche of its Series B round of financing by selling an aggregate of 7,519,592 shares of Series B convertible preferred stock at $12.125 per share for gross proceeds of $91.2 million, with an additional $9.0 million of Series B convertible preferred stock to be sold to two investors within 180 days of the first tranche closing at the issuance price per share of the Series B convertible preferred stock. The Company accounted for this issuance as forward options to issue shares at a fixed price. As the forward options expired in 180 days, and there was limited expected volatility in the Series B convertible preferred stock issuance price, the value of the forward options was considered immaterial at December 31, 2016. In March 2017, the Company issued an In June 2017, the Company closed the second and final tranche of its Series B convertible preferred stock round of financing by selling an aggregate of 2,879,288 shares of Series B convertible preferred stock at $12.125 per share for gross proceeds of $34.9 million. Included in the terms of the Series B Preferred Stock Agreement were rights to purchase additional tranches of Series B convertible preferred stock under the same terms as those provided at the initial closing. The Company did not separately account for these tranche purchase rights as a forward option as neither the purchasers nor the Company had a commitment or obligation to purchase or sell additional shares until the tranche closing occurred. In March 2018, the Company amended and restated its certificate of incorporation to, among other things, (i) increase its authorized shares of common stock from 122,000,000 to 140,000,000 shares, (ii) increase its authorized shares of preferred stock from 91,739,149 to 103,283,818 shares, of which 11,544,669 shares were designated as Series C convertible preferred stock, and (iii) set forth the rights, preferences and privileges of the Series C convertible preferred stock. In March 2018, the Company sold 3,590,573 shares of Series C convertible preferred stock at $15.3317 per share for net proceeds of $54.9 million and in Each share of Series C convertible preferred stock was convertible into one share of the Company’s common stock. Each share of preferred stock was automatically converted into one share of common stock upon the consummation of a qualified public offering. A qualified public offering was defined as an initial public offering that resulted in listing on a U.S. national securities exchange and at least $30.0 million of gross proceeds at a per share price of not less than the Series C original issue price of $15.3317. The Company evaluated the other rights, preferences and privileges of each series of convertible preferred stock and concluded that there were no freestanding derivative instruments or any embedded derivatives requiring bifurcation. Upon the closing of the IPO, all of the Company’s outstanding shares of convertible preferred stock were converted into 32,073,149 shares of common stock. In addition, all 763,501 of the Company’s convertible preferred stock warrants were converted into warrants to purchase shares of common stock. As of December 31, 2018, the Company had no shares of convertible or preferred stock issued or outstanding. As of December 31, 2017, convertible preferred stock consisted of the following (in thousands, except share amounts): Shares Authorized Shares Issued and Outstanding Liquidation Preference Carrying Value Series A-1 9,085,738 2,887,086 $ 2,495 $ 2,457 Series A-2 32,653,411 10,498,269 9,198 9,214 Series B 50,000,000 14,774,369 179,132 162,285 Total convertible preferred stock 91,739,149 28,159,724 $ 190,825 $ 173,956 Prior to the conversion of the convertible preferred stock upon closing of the IPO, the rights, preferences and privileges of the convertible preferred stock were as follows: Conversion Rights Each share of convertible preferred stock was convertible at the right and option of the stockholder, at any time after the date of issuance, into such number of fully paid and non-assessable shares of common stock on a one for one ratio (1:1 conversion ratio). The Series A-1 conversion price was $0.864 per share, the Series A-2 conversion price was $0.876 per share, the Series B conversion price was $12.125 per share and the Series C conversion price was $15.3317 per share, in each case, subject to certain antidilution adjustments as provided in the Company’s amended and restated certificate of incorporation. Each share of convertible preferred stock was automatically convertible into a fully paid, non-assessable share of common stock at the then-effective conversion rate for such share (i) upon the closing of a firm commitment, underwritten initial public offering of the Company’s common stock with aggregate gross proceeds of not less than $30.0 million and a price per share to the public of not less than $15.3317 per share; or (ii) upon the receipt by the Company of a written request for such conversion from at least 60% of the holders of the convertible preferred stock then outstanding (voting together as a single class and on an as-converted basis), or if later, the effective date for conversion specified in such requests. Liquidation Rights In the event of any voluntary or involuntary liquidation, dissolution or winding up of the Company or a deemed liquidation event, as further defined in the Company’s amended and restated certificate of incorporation, prior to and in preference to any distribution of any of the assets of the Company to the holders of Series B convertible preferred stock and the Series A-1 and Series A-2 convertible preferred stock and common stock, the holders of Series C convertible preferred stock would have been paid, on a pari passu After the payment or setting aside for payment to the holders of the Series C convertible preferred stock of the full amount of the Series C Liquidation Preference, prior to any distribution of any of the assets of the Company to the holders of the Series A-1 and Series A-2 convertible preferred stock and common stock, the holders of Series B convertible preferred stock would have been paid, on a pari passu After the payment or setting aside for payment to the holders of the Series B convertible preferred stock of the full amount of the Series B Liquidation Preference, prior to any distribution of any of the assets of the Company to the holders of the common stock, the holders of Series A-1 and Series A-2 convertible preferred stock would have been paid, on a pari passu basis, an amount per share equal to $0.864 per share for Series A-1 and $0.876 per share for Series A-2, plus, in each case, an amount equal to any dividends declared but unpaid thereon (the “Series A Liquidation Preference”). If upon any such liquidation, dissolution or winding up of the Company or deemed liquidation event, the assets of the Company available for distribution to its stockholders had been insufficient to pay the holders of shares of Series A-1 and Series A-2 convertible preferred stock the full amount to which they shall be entitled, the holders of the Series A-1 and Series A-2 convertible preferred stock would have shared ratably in any distribution of the assets available for distribution in proportion to the respective amounts which would otherwise have been payable in respect of the shares held by them upon such distribution if all amounts payable on or with respect to such shares were paid in full. After the payments or setting aside for payment to the holders of convertible preferred stock of the full amounts specified above, the entire remaining assets of the Company legally available for distribution shall be distributed pro rata to holders of the common stock of the Company in proportion to the number of shares of common stock held by them. Voting Rights The holders of outstanding shares of Series A-1 and Series A-2 convertible preferred stock, voting together as a single class, were entitled to elect two members of the Company’s Board of Directors. The holders of outstanding shares of Series B convertible preferred stock, voting together as a single class, were entitled to elect one member of the Company’s Board of Directors. Additionally, each holder of the Company’s convertible preferred stock was entitled to a vote equal to the number of shares of common stock into which the shares of convertible preferred stock could have been converted as of the record date. The holders of convertible preferred were entitled to vote on all matters on which the common stock shall be entitled to vote. Dividend Rights Holders of the Series A-1, Series A-2, Series B and Series C convertible preferred stock were entitled to receive non-cumulative dividends at a rate of 6% of the original respective series of convertible preferred stock issuance price. Only after payment of the dividends to the holders of Series C convertible preferred stock were the holders of shares of Series B, Series A-1 and Series A-2 convertible preferred stock be entitled to receive dividends, out of any assets legally available therefore, prior and in preference to any declaration or payment of any dividend (other than dividends on the common stock payable solely in common stock) on the common stock. After the payment or setting aside for payment of the dividends described above, any additional dividends (other than dividends on common stock payable solely in common stock) set aside or paid in any fiscal year could have been set aside or paid among the holders of the convertible preferred stock and common stock then outstanding on a pari passu basis in proportion to the greatest whole number of shares of common stock which would have been held by each such holder if all shares of convertible preferred stock were converted at the then-effective conversion rate. Dividends were only payable as and if declared by the Board of Directors. To date, the Company has not declared or paid any dividends. Redemption Rights The convertible preerred stock was not mandatorily redeemable as it did not have a set redemption date or a date after which the shares may be redeemed by the holders. A redemption event would have occurred only upon the occurrence of certain change in control events that are outside the Company’s control, including a sale, lease, transfer, or other disposition of all or substantially all of the Company’s assets. The Company has elected not to adjust the carrying values of the convertible preferred stock to the liquidation preferences of such shares because it is uncertain whether or when an event would have occurred that would obligate the Company to pay the liquidation preferences to holders of shares of convertible preferred stock. Subsequent adjustments to the carrying values of the liquidation preferences will be made only when it becomes probable that such a liquidation event will occur.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	10. Stock-Based Compensation Equity Incentive Plans In March 2018, the Company’s board of directors adopted the Company’s 2018 Incentive Award Plan (the “2018 Plan”). The 2018 Plan was approved by the Company’s stockholders in April 2018 and became effective on May 2, 2018. The 2018 Plan initially reserved 4,289,936 shares for the issuance of stock options as well as any automatic annual increases in the number of shares of common stock reserved for future issuance under the 2018 Plan. Awards granted under the 2018 Plan expire no later than ten years from the date of grant. For stock options, the option price shall not be less than 100% of the estimated fair value on the day of grant. Options granted typically vest over a four-year period but may be granted with different vesting terms. Unvested options not exercised at the time of an employee’s termination of employment are added back to the 2018 Plan. Following the Company’s IPO and in connection with the effectiveness of the 2018 Plan, the 2013 Equity Incentive Plan (the “2013 Plan”) terminated and no further awards will be granted under that plan. All outstanding awards under the 2013 Plan will continue to be governed by their existing terms and the shares that remained outstanding for issuance under the 2013 Plan were transferred into the 2018 Plan. As of December 31, 2018, there was an aggregate 5,058,434 shares of common stock authorized for issuance under the 2018 Plan. Prior to its termination, the 2013 Plan provided for the granting of incentive stock options (“ISOs”), non-statutory stock options (“NSOs”) and restricted shares to employees, directors, and consultants at the discretion of management and the Board of Directors. The exercise price of an ISO and NSO shall not be less than 100% of the estimated fair value of the shares on the date of grant, and the exercise price of an ISO and NSO granted to a 10% stockholder shall not be less than 110% of the estimated fair value of the shares on the date of grant. For awards granted between September 2017 and February 2018 with an exercise price of $3.42, a deemed fair value ranging from $3.95 to $8.47 per share was used in calculating stock-based compensation expense, which was determined using management hindsight. Options granted under the 2013 Plan expire no later than 10 years from the date of grant and generally vest over a four-year period but may be granted with different vesting terms. Unvested options not exercised at the time of an employee’s termination of employment are added back to the 2018 Plan. Under the 2013 Plan, the Company permited early exercise of certain stock options prior to vesting. These unvested shares are subject to repurchase by the Company at the original issuance price in the event the optionee’s employment is terminated either voluntarily or involuntarily. The amounts paid for shares purchased under an early exercise of stock options and subject to repurchase by the Company are reported as a liability and reclassified into additional paid-in capital as the shares vest. Stock Option Activity A summary of the Company’s stock option activity under the 2013 and 2018 Plan is as follows: Shares Available for Grant Outstanding Options Weighted- Average Exercise Price Weighted- Average Remaining Contract Term Aggregate Intrinsic Value (in Years) (in thousands) Balances at December 31, 2017 918,595 4,196,213 $ 3.06 Retired from 2013 Plan (378,875 ) — — Authorized 4,289,936 — — Granted (2,082,265 ) 2,082,265 13.20 Exercised — (501,329 ) 3.17 Canceled 280,411 (276,618 ) 5.82 Balances at December 31, 2018 3,027,802 5,500,531 $ 6.75 8.49 $ 53,051 Vested and exercisable at December 31, 2018 1,619,419 $ 2.72 7.65 $ 21,929 Vested and expected to vest at December 31, 2018 5,500,531 $ 6.75 8.49 $ 53,051 The total intrinsic value of options exercised was $1.5 million, $0.1 million and $20,000 for the years ended December 31, 2018, 2017 and 2016, respectively. The weighted-average estimated fair value of stock options granted was $13.20, $3.40 and $0.32 for the years ended December 31, 2018, 2017 and 2016, respectively. The aggregate intrinsic value of options exercisable was $21.9 million and $3.3 million as of December 31, 2018 and 2017, respectively. As of December 31, 2018, the total stock-based compensation cost related to options granted but not yet amortized was $20.6 million and will be recognized over a weighted-average period of approximately 4.1 years. The total grant-date fair value of stock options granted to employees that vested during the years ended December 31, 2018 and 2017 was approximately $3.5 million and $1.5 million. respectively. Stock Options Granted to Employees with Service-Based Vesting The fair value of stock options granted to employees was estimated on the date of grant using the Black-Scholes option pricing model using the following assumptions: Year Ended December 31, 2018 2017 2016 Expected dividend yield — — — Expected term of options (in years) 6.1 5.6–6.7 5.3–6.1 Risk-free interest rate 2.6%-3.0% 1.8%–2.2% 1.2%–2.1% Expected stock price volatility 87.4%-92.6% 77.0%–82.0% 76.1%–79.7% The valuation assumptions were determined as follows: Expected Term —The expected term represents the period that the options granted are expected to be outstanding and is determined using the simplified method (based on the mid-point between the vesting date and the end of the contractual term) as the Company has concluded that its stock option exercise history does not provide a reasonable basis upon which to estimate expected term. Expected Volatility —The Company used an average historical stock price volatility of comparable public companies within the biotechnology and pharmaceutical industry that were deemed to be representative of future stock price trends as the Company does not have a sufficient historical trading history for its own common stock. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available. Risk-Free Interest Rate —The Company based the risk-free interest rate over the expected term of the options based on the constant maturity rate of U.S. Treasury securities with similar maturities as of the date of the grant. Expected Dividends —The Company has never paid any dividends and does not plan to pay dividends in the foreseeable future. Therefore, the expected dividend yield is zero. Performance Contingent Stock Options Granted to Employees During the year endend December 31, 2018, the Board of Directors granted performance contingent stock option awards exercisable for 53,575 shares, to certain members of the Company’s executive team. These awards had a weighted average exercise price of $3.42 which was based on the fair market value on the grant date, as determined by the Board of Directors, and vest upon the successful achievement of one or more specified performance goals. The total estimated fair value of employee performance contingent stock option awards was $0.4 million and was estimated at the date of grant using a Black-Scholes option-pricing model using the same assumptions as the stock options granted to employees with service-based vesting conditions. As of December 31, 2018, and 2017, there were 329,498 and 275,922 performance contingent stock option awards outstanding with a total grant date fair value of $0.7 million and $0.3 million respectively. As of December 31, 2018, and 2017, the Company determined that the achievement of the requisite performance conditions was not probable and, as a result, no compensation cost was recognized for these awards. Performance and Market Contingent Stock Options Granted to Employees During the year ended December 31, 2018, the Board of Directors granted performance and market contingent stock option awards exercisable for 160,727 shares of common stock to certain members of the Company’s executive team. These awards had a weighted average exercise price of $3.42, which was based on the fair market value on the grant date, as determined by the Board of Directors. The total estimated grant-date fair value of these options was $0.7 million. Key assumptions in the valuation model included expected volatility, a risk-free interest rate, expected dividend yield, and an expected term unique to the terms of these awards. Under the performance and market contingent awards, 53,575 of the shares have three separate market triggers for vesting based upon (i) the closing of a financing where the Company sells shares of its equity securities to institutional investors at a minimum price per share, (ii) a change in control with aggregate proceeds payable for the Company’s common stock at a minimum price per share, or (iii) an initial public offering that becomes effective at a minimum specified price per share. The remaining 107,152 shares have three separate market triggers for vesting based upon (i) the closing of a financing where the Company sells shares of its equity securities to institutional investors at a minimum pre-money valuation, (ii) a change in control with minimum aggregate proceeds payable for the Company’s common stock at a minimum price per share, or (iii) either an initial public offering or an achievement of a minimum market capitalization, as measured by a trailing 30 day volume-weighted average price. By definition, the market condition in these awards can only be achieved after the performance condition of a liquidity event has been achieved. As such, the requisite service period is based on the estimated period over which the market condition can be achieved. When a performance goal is deemed to be probable of achievement, which for liquidity events is generally upon achievement, time-based vesting and recognition of stock-based compensation expense commence. As of December 31, 2018 and 2017, there were 454,584 and 360,594 performance and market contingent stock option awards outstanding with a grant date total fair value of $1.0 million and $0.4 million respectively. As of December 31, 2018 and 2017, the Company determined that the achievement of the requisite performance conditions was not probable and, as a result, no compensation cost was recognized for these awards. Stock-Based Compensation for Nonemployees The Company has granted options to purchase shares of common stock to consultants in exchange for services performed. During the years ended December 31, 2018 and 2017, the Company granted options to purchase an aggregate of 20,337 and 235,250 shares (of which an aggregate of 169,491 were issued outside of the 2018 and 2013 Plan The fair value of stock options granted to nonemployees was estimated on the date of grant using the Black-Scholes option pricing model. The valuation assumptions used were substantially consistent with the assumption used to value the employee options with the exception of the expected term which was based on the contractual term of the award. During the years ended December 31, 2018 and 2017, stock-based compensation expense recognized related to nonemployee options was $1.2 million and $0.4 million, respectively. Restricted Stock A summary of the Company’s restricted stock activity for the year ended December 31, 2018 was as follows: Shares Weighted- Average Grant Date Fair Value Unvested at December 31, 2017 478,971 $ 4.57 Granted — $ 4.57 Vested (119,742 ) $ 4.57 Unvested at December 31, 2018 359,229 $ 4.57 In October 2017, the Company and an executive officer entered into two restricted stock agreements whereby the executive officer purchased an aggregate of 625,084 shares of restricted stock of which 146,113 shares vested immediately, 119,742 shares vest on January 1, 2018 and 359,229 shares vest on January 1, 2019. As discussed in Note 8, the purchase of the restricted stock was through the issuance of promissory notes which were considered to be non-recourse in substance and accordingly, considered an option for accounting purposes. The Company measured compensation cost for this option based on its fair value on the grant date using the Black-Scholes option pricing model considering an expected term commensurate with the expected timing to a liquidity event which would trigger repayment of these promissory notes and an exercise price consistent with the repayment term of the promissory notes. The Company recognized compensation cost over the requisite service period with an offsetting credit to additional paid-in capital. The shares of restricted stock have only been included in the shares issued and outstanding as such shares are legally issued. 2018 Employee Stock Purchase Plan In March 2018, the Company’s board of directors adopted the Company’s 2018 Employee Stock Purchase Plan (the “2018 ESPP”). The 2018 ESPP was approved by the Company’s stockholders in April 2018 and became effective on May 2, 2018. The 2018 ESPP reserved 536,242 shares of common stock for issuance pursuant to future awards, as well as any automatic increases in the number of shares of our common stock reserved for future issuance under this plan. Under the 2018 ESPP, employees are offered the option to purchase the Company’s common stock at a discount during the offering periods, at semi-annual intervals, with their accumulated payroll deductions. The option purchase price will be 85% of the lower of the closing trading price per share at the beginning of the offering period or at the purchase date. The 2018 ESPP provides for consecutive offering periods and eligible employees may elect to withhold up to 15% of their compensation through payroll deductions during the offering period for the purchase of stock. The maximum number of shares that may be purchased by any one participant is limited to 15,000 shares in each offering period and $25,000 in fair market value during any calendar year per the Internal Revenue Code limits. The first offering period commenced on September 16, 2018. The fair values of the rights granted under the 2018 ESPP were calculated using the following assumptions: Year Ended December 31, 2018 Expected dividend yield — Expected term of options (in years) 0.7 Risk-free interest rate 2.41% Expected stock price volatility 73.18% Stock-Based Compensation Expense The following table sets forth the total stock-based compensation expense for all options granted to employees and nonemployees, including shares sold through the issuance of non-recourse promissory notes which are considered to be options for accounting purposes, and costs associated with the Company’s 2018 ESPP included in the Company’s statement of operations (in thousands): Year Ended December 31, 2018 2017 2016 Research and development $ 6,043 $ 1,695 $ 164 General and administrative 3,398 1,339 60 Total $ 9,441 $ 3,034 $ 224

Warrants

Warrants	12 Months Ended
	Dec. 31, 2018
Warrants And Rights Note Disclosure [Abstract]
Warrants	11. Warrants In June 2013, the Company granted warrants to its then Chief Executive Officer (“CEO”), considered to be a related party, to purchase 192,823 shares of Series A-1 convertible preferred stock with an exercise price of $0.65 per share and 190,226 shares of Series A-2 convertible preferred stock at a price of $0.66 per share as compensation. In January 2015, the Company granted warrants to the aforementioned CEO to purchase an aggregate of 380,452 shares of Series A-2 convertible preferred stock with an exercise price of $0.66 per share as compensation. Upon the completion of the IPO, these warrants converted to common stock warrants. These warrants were exercisable beginning on January 1, 2018 and expired on the earlier of (i) December 31, 2018, (ii) December 31 of the year in which a change of control occurs or (iii) December 31 of the year in which the holder terminates service. All of the vested warrants expired unexercised on December 31, 2018. In October 2013, the Company granted warrants to a nonemployee to purchase an aggregate of 96,610 shares of common stock with an exercise price of $0.18 per share of which 9,425 warrants vested immediately. During April 2018 the nonemployee exercised the vested shares and the remaining unvested warrants expired on May 3, 2018 upon the closing of the IPO.

Net Loss per Common Share

Net Loss per Common Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Net Loss per Common Share	12. Net Loss per Common Share Basic net loss per share is calculated by dividing net loss by the weighted average number of shares outstanding for the period. Diluted net loss per share is calculated by dividing net loss by the weighted average number of shares of common stock and potential dilutive common stock equivalents outstanding during the period if the effect is dilutive. The calculation of diluted earnings (loss) per share also requires that, to the extent the presumed issuance of additional shares as contingent consideration is dilutive to earnings (loss) per share for the period, adjustments to net income or net loss used in the calculation are required to remove the change in fair value of the contingent consideration liability for the period. Likewise, adjustments to the denominator are required to reflect the related dilutive shares. In all periods presented, the Company’s outstanding stock options, convertible preferred stock, early exercised common stock subject to future vesting, restricted stock accounted for as options common and preferred stock warrants, shares subject to the 2018 ESPP and presumed issuance of additional shares as contingent consideration were excluded from the calculation of diluted net loss per share because their effects were antidilutive. A reconciliation of the numerators and denominators used in computing net loss from continuing operations per share is as follows (in thousands, except per share amounts): December 31, 2018 2017 2016 (in thousands, except share and per share amounts) Numerator: Net loss $ (76,398 ) $ (44,656 ) $ (30,404 ) Denominator: Weighted average number of shares outstanding—basic and diluted 28,269,907 3,197,516 2,662,841 Net loss per share—basic and diluted $ (2.70 ) $ (13.97 ) $ (11.42 ) Since the Company was in a loss position for all periods presented, basic net loss per common share is the same as diluted net loss per common share as the inclusion of all potential common shares outstanding would have been anti-dilutive. Potentially dilutive securities that were not included in the diluted per share calculations because they would be anti-dilutive were as follows: Year Ended December 31, 2018 2017 2016 Convertible preferred stock — 28,159,724 24,620,615 Options to purchase common stock 5,500,531 4,365,694 508,418 Early exercised common stock subject to future vesting 704,028 831,439 1,287,435 Restricted stock accounted for as options 359,228 625,084 — Warrants to purchase convertible preferred stock — 763,501 763,501 Warrants to purchase common stock — 96,610 96,610 Shares subject to the 2018 ESPP 27,622 — — Total 6,591,409 34,842,052 27,276,579 Up to 606,218 shares may be contingently issued, if certain performance conditions are met under the Company’s in-licensing agreements .

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2018
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan	13. Defined Contribution Plan The Company sponsors a 401(k) Plan that stipulates that eligible employees can elect to contribute to the 401(k) Plan, subject to certain limitations, on a pretax basis. The Company does not match any employee contributions. In January 2019, the Company began to match 4% of employees’ salary.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	14. Income Taxes The Company has incurred net operating losses for all the periods presented. The Company has not reflected the benefit of any such net operating loss carryforwards in the accompanying financial statements. The Company has established a full valuation allowance against its deferred tax assets due to the uncertainty surrounding the realization of such assets. All losses to date have been incurred domestically as the Company has no international operations or subsidiaries. No provision for U.S. income taxes exists due to tax losses incurred in all periods presented. All losses incurred were U.S. based. The effective tax rate for the years ended December 31, 2018, 2017 and 2016 is different from the federal statutory rate primarily due to the valuation allowance against deferred tax assets as a result of insufficient sources of income. The effective tax rate of the provision for income taxes differs from the federal statutory rate as follows: Year Ended December 31, 2018 2017 2016 Taxes at the U.S. statutory income tax rate 21.0 % 34.0 % 34.0 % State tax, net of federal benefit 0.9 — — Other (0.1 ) (2.2 ) — Stock-based compensation 0.3 — — General business credits 1.0 — — Change in valuation allowance (23.1 ) (13.3 ) (21.0 ) Non-deductible interest expense — — (13.0 ) Change in income tax rate due to Tax Act — (18.5 ) — Total provision for income taxes — % — % — % On December 22, 2017, the U.S. federal government enacted the Tax Cuts and Jobs Act (Tax Act). The Tax Act contains, among other things, significant changes to corporate taxation, including reduction of the corporate tax rate from a top marginal rate of 35% to a flat rate of 21% for tax years beginning after December 31, 2017, limitation of the deduction for net operating losses to 80% of current year taxable income and elimination of net operating loss carrybacks, implementing a territorial tax system, and requiring a mandatory one-time tax on U.S. owned undistributed foreign earnings and profits known as the transition tax. Pursuant to SAB 118, an entity may select between one of three scenarios to determine a reasonable estimate arising from the Tax Act. The scenarios are (i) a final estimate which effectively closes the measurement window; (ii) a reasonable estimate leaving the measurement window open for future revisions; and (iii) no estimate as the law is still being analyzed. The Company was able to provide a reasonable estimate for the revaluation of deferred taxes. As such, the Company recorded a $8.3 million reduction in deferred tax assets for the revaluation of deferred taxes in 2017 which was offset by a corresponding decrease to the Company’s full valuation allowance. The ultimate impact of the Act did not differ materially from provision amounts recorded. Adjustments, if any, would not have impacted the statement of operations and comprehensive loss due to the full valuation allowance on the Company’s deferred tax assets. The tax effects of significant items comprising the Company’s deferred tax assets are as follows: December 31, 2018 2017 (in thousands) Deferred tax assets: Net operating loss $ 29,926 $ 16,530 Research and development credits 3,865 1,879 Stock-based compensation 1,839 671 Contingent consideration 954 — Accruals and other 1,040 895 Charitable contributions 253 330 Total deferred tax assets 37,877 20,305 Valuation allowance (37,877 ) (20,236 ) Net deferred tax assets — 69 Deferred tax liability — (69 ) Net deferred tax assets $ — $ — ASC 740 requires that the tax benefit of net operating losses, temporary differences and credit carryforwards be recorded as an asset to the extent that management assesses that realization is "more likely than not." Realization of the future tax benefits is dependent on the Company's ability to generate sufficient taxable income within the carryforward period. Because of the Company's recent history of operating losses, management believes that recognition of the deferred tax assets arising from the above-mentioned future tax benefits is currently not likely to be realized and, accordingly, has provided a valuation allowance. Realization of the net deferred tax assets is dependent upon future taxable income, if any, the amount and timing of which is uncertain. Based on the weight of available positive and negative objective evidence, management believes it more likely than not that the Company’s deferred tax assets are not realizable. Accordingly, the net deferred tax assets have been fully offset by a valuation allowance. Net operating losses and tax credit carryforwards as of December 31, 2018 are as follows: Amount Expiration Years Net operating losses, federal (post December 31, 2017) $ 64,593 indefinite Net operating losses, federal (pre January 1, 2018) 64,136 2030 - 2038 Net operating losses, state 64,663 2030 - 2039 Tax credits, federal 2,988 2031 - 2039 Tax credits, state 2,692 Indefinite The net operating loss and research and development credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service (“IRS”) and state tax authorities and may become subject to an annual limitation in the event of certain future cumulative changes in the ownership interest of significant stockholders over a three-year period in excess of 50%, as defined under Sections 382 and 383 of the Internal Revenue Code of 1986. The Company has performed this analysis and concluded that an additional $1.6 million of net operating losses and research development credits, collectively, were limited under Section 382, which has been reflected in the amounts disclosed in the financials. The Company determines its uncertain tax positions based on a determination of whether and how much of a tax benefit taken by the Company in its tax filings is more likely than not to be sustained upon examination by the relevant income tax authorities. A reconciliation of the beginning and ending amounts of unrecognized tax benefits is as follows: December 31, 2018 2017 (in thousands) Gross unrecognized tax benefits at January 1 $ 3,065 $ 2,800 Additions for tax positions taken in the current year 753 478 Reductions for tax positions taken in the prior year (104 ) (213 ) Gross unrecognized tax benefits at December 31 $ 3,714 $ 3,065 If recognized, none of the unrecognized tax benefits as of December 31, 2018 and 2017 would reduce the annual effective tax rate, primarily due to corresponding adjustments to the valuation allowance. The Company will recognize both accrued interest and penalties related to unrecognized benefits in income tax expense. As of December 31, 2018 and 2017, no liability has been recorded for potential interest or penalties. The Company does not expect the unrecognized tax benefits to change significantly over the next 12 months. Since the Company is in a loss carryforward position, the Company is generally subject to examination by the U.S. federal, state and local income tax authorities for all tax years in which a loss carryforward is available.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	15. Subsequent Events In February 2019, the Company entered into a lease agreement for approximately 62,655 square feet of office, research and development and laboratory space in South San Francisco, California. The lease is expected to commence on October 1, 2019. The lease has an approximately ten year term with an option to extend for a period of eight years subject to certain conditions. Pursuant to the lease agreement, the Company provided a $0.9 million letter of credit to the landlord for the term of the lease.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data (Unaudited)	16. Selected Quarterly Financial Data (Unaudited) The following tables show a summary of the Company’s quarterly financial information for each of the four quarters of 2018 and 2017 and has been prepared in accordance with GAAP for interim financial reporting (in thousands, except for per share data): Quarter Year Ended December 31, 2018 First Second Third Fourth Loss from operations $ (16,482 ) $ (20,798 ) $ (19,377 ) $ (22,808 ) Net loss $ (16,133 ) $ (20,002 ) $ (18,346 ) $ (21,917 ) Net loss per common share, basic and diluted $ (4.69 ) $ (0.76 ) $ (0.45 ) $ (0.53 ) Quarter Year Ended December 31, 2017 First Second Third Fourth Loss from operations $ (9,040 ) $ (11,698 ) $ (12,083 ) $ (12,787 ) Net loss $ (8,934 ) $ (11,428 ) $ (11,750 ) $ (12,544 ) Net loss per common share, basic and diluted $ (2.90 ) $ (3.62 ) $ (3.63 ) $ (3.67 )

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Need for Additional Capital	Need for Additional Capital The accompanying financial statements have been prepared assuming that the Company will continue as a going concern. Since inception, the Company has incurred net losses and negative cash flows from operations. During the year ended December 31, 2018, the Company incurred a net loss of $76.4 million and used $56.6 million of cash in operations. At December 31, 2018, the Company had an accumulated deficit of $163.3 million and does not expect positive cash flows from operations in the foreseeable future. The Company has historically financed its operations primarily through the issuance and sale of convertible preferred stock and convertible promissory notes. To date, none of the Company’s drug candidates have been approved for sale and therefore the Company has not generated any revenue from contracts with customers. The Company has evaluated and concluded there are no conditions or events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern for a period of one year following the date that these financial statements are issued. Management expects operating losses to continue for the foreseeable future. As a result, the Company will need to raise additional capital. If sufficient funds on acceptable terms are not available when needed, the Company could be required to significantly reduce its operating expenses and delay, reduce the scope of, or eliminate one or more of its development programs. Failure to manage discretionary spending or raise additional financing, as needed, may adversely impact the Company’s ability to achieve its intended business objectives.
Basis of Presentation	Basis of Presentation These financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”).
Reverse Stock Split	Reverse Stock Split On April 19, 2018, the Company’s board of directors approved an amendment to the Company’s amended and restated certificate of incorporation to effect a 1-for-2.95 reverse split (“Reverse Split”) of shares of the Company’s common and convertible preferred stock, which was effected on April 20, 2018. The par value and authorized shares of common stock and convertible preferred stock were not adjusted as a result of the Reverse Split. All of the share and per share information included in the accompanying financial statements has been adjusted to reflect the Reverse Split.
Initial Public Offering	Initial Public Offering On May 7, 2018, the Company closed its initial public offering (“IPO”), of 5,000,000 shares of common stock, at an offering price to the public of $17.00 per share. The Company received net proceeds of approximately $75.9 million, after deducting underwriting discounts, commissions and offering related transaction costs of approximately $9.1 million. In connection with the IPO, all of the Company’s outstanding shares of convertible preferred stock were automatically converted into 32,073,149 shares of common stock. In addition, all of the Company’s convertible preferred stock warrants were converted into warrants to purchase shares of common stock. In connection with the completion of its IPO, on May 7, 2018, the Company’s certificate of incorporation was amended and restated to provide for 300,000,000 authorized shares of common stock with a par value of $0.0001 per share and 10,000,000 authorized shares of preferred stock with a par value of $0.0001 per share.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. The Company bases its estimates on historical experience and market-specific or other relevant assumptions that it believes are reasonable under the circumstances. The amounts of assets and liabilities reported in the Company’s balance sheets and the amount of expenses and income reported for each of the periods presented are affected by estimates and assumptions, which are used for, but are not limited to, determining the fair value of assets and liabilities, contingent consideration liability, common stock valuation, and stock-based compensation. Actual results could differ from such estimates or assumptions.
Segments	Segments The Company has one operating segment. The Company’s chief operating decision maker, its Chief Executive Officer, manages the Company’s operations on a consolidated basis for the purposes of allocating resources.
Cash, Cash Equivalents and Restricted Cash	Cash, Cash Equivalents and Restricted Cash The Company considers all highly liquid investments with original maturities of 90 days or less from the date of purchase to be cash equivalents. Cash equivalents primarily include money market funds that invest in U.S. Treasury obligations which are stated at fair value. The Company has issued a letter of credit under a lease agreement which has been collateralized. This cash is classified as noncurrent restricted cash on the balance sheet based on the term of the underlying lease. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets that sum to the total of the same amounts shown in the statements of cash flows. December 31, 2018 2017 2016 (in thousands) Cash and cash equivalents $ 15,399 $ 7,298 $ 89,286 Restricted cash 550 550 450 Total cash, cash equivalents, and restricted cash $ 15,949 $ 7,848 $ 89,736
Marketable Securities	Marketable Securities The Company generally invests its excess cash in investment grade, short to intermediate-term, fixed income securities. Such investments are considered available-for-sale, and reported at fair value with unrealized gains and losses included as a component of stockholders’ deficit. Marketable securities with original maturities of greater than 90 days from the date of purchase but less than one year from the balance sheet date are classified as short-term, while marketable securities with maturities in one year or beyond one year from the balance sheet date are classified as long-term. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity, which is included in interest income on the statements of operations and comprehensive loss. Realized gains and losses and declines in value judged to be other-than-temporary, if any, on marketable securities are included in interest income (expense), net. The cost of securities sold is determined using the specific identification method. The Company periodically evaluates whether declines in fair values of its marketable securities below their book value are other-than-temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the marketable security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any marketable securities before recovery of its amortized cost basis. Factors considered include quoted market prices, recent financial results and operating trends, implied values from any recent transactions or offers of investee securities, credit quality of debt instrument issuers, other publicly available information that may affect the value of the marketable security, duration and severity of the decline in value, and management’s strategy and intentions for holding the marketable security. To date, the Company has not recorded any impairment charges on its marketable securities related to other-than-temporary declines in market value.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company’s financial instruments during the periods presented consist of cash and cash equivalents, restricted cash, contribution receivable, marketable securities, prepaid expenses and other current assets, accounts payable, accrued compensation, accrued and other current liabilities. Fair value estimates of these instruments are made at a specific point in time, based on relevant market information. These estimates may be subjective in nature and involve uncertainties and matters of significant judgment.
Concentration of Risk	Concentrations of Risk Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash, cash equivalents, restricted cash, marketable securities and contribution receivable. Substantially all of the Company’s cash and cash equivalents and restricted cash is deposited in accounts with financial institutions that management believes are of high credit quality. Such deposits have and will continue to exceed federally insured limits. The Company maintains its cash with accredited financial institutions and accordingly, such funds are subject to minimal credit risk. The Company has not experienced any losses on its cash deposits. The contribution receivable is unsecured and is concentrated with one third-party organization, and accordingly the Company may be exposed to credit risk. To date, the Company has not experienced any loss related to its contributions receivable. The Company’s investment policy limits investments to certain types of securities issued by the U.S. government, its agencies and institutions with investment-grade credit ratings and places restrictions on maturities and concentration by type and issuer. The Company is exposed to credit risk in the event of a default by the financial institutions holding its cash, cash equivalents, restricted cash and marketable securities and issuers of marketable securities to the extent recorded on the balance sheets. As of December 31, 2018, the Company had no off-balance sheet concentrations of credit risk. The Company depends on third-party suppliers for key raw materials used in its manufacturing processes and is subject to certain risks related to the loss of these third-party suppliers or their inability to supply the Company with adequate raw materials.
Contribution Revenue and Receivables	Contribution Revenue and Receivables The Company recognizes contribution revenue related to the receipt of cash from third-party resource providers not considered to be customers and where the transfer of assets is not an exchange transaction or financing of research and development. Contribution revenue and related receivables are recognized for conditional contributions as the conditions related to the contribution are relieved. In July 2017, the Company entered an arrangement with a third-party organization under which the Company would be provided with up to $1.5 million of funding for the performance of certain research and development activities during the 90-day period following the arrangement in pursuit of the third-party organization’s philanthropic mission. All conditions related to this contribution were met during 2017 and the Company recognized $1.4 million under this arrangement, which was recorded as contribution revenue in the statement of operations and a contribution receivable on the balance sheet.
Research and Development Expenses	Research and Development Expenses Costs related to research, design and development of drug candidates are charged to research and development expense as incurred. Research and development costs include, but are not limited to, payroll and personnel expenses for personnel contributing to research and development activities, laboratory supplies, outside services, licenses acquired to be used in research and development and allocated overhead, including rent, equipment, depreciation and utilities. Payments made prior to the receipt of goods or services to be used in research and development are deferred and recognized as expense in the period in which the related goods are received or services are rendered. Such payments are evaluated for current or long-term classification based on when they will be realized.
Contingent Consideration Liability	Contingent Consideration Liability The Company has entered into and may continue to enter into, license agreements to access and utilize certain technology. In each case, the Company evaluates if the license agreement results in the acquisition of an asset or a business. To date none of the Company’s license agreements have been considered an acquisition of a business. For asset acquisitions, the upfront payments to acquire such licenses, as well as any future milestone payments made before product approval, are immediately recognized as research and development expense when due, provided there is no alternative future use of the rights in other research and development projects. These license agreements also include contingent consideration in the form of additional issuances of the Company’s common stock based on the achievement of certain milestones. For asset acquisitions, the Company assesses on a continuous basis whether such contingent consideration meets the definition of a derivative and can or cannot be classified within stockholders’ equity, until such time that equity classification criteria are met or the milestones expire. The derivative related to this contingent consideration is measured at fair value as of each balance sheet date with the related change in fair value being reflected in operating expenses. Upon a reassessment event that results in the contingent consideration no longer meeting the definition of a derivative and/or meeting equity classification critera, the final change in fair value of the instrument is recorded within operating expenses and the liability is reclassified into stockholders’ equity.
Variable Interest Entities	Variable Interest Entities The Company reviews agreements it enters into with third-party entities, pursuant to which the Company may have a variable interest in the entity, in order to determine if the entity is a variable interest entity (“VIE”). If the entity is a VIE, the Company assesses whether or not it is the primary beneficiary of that entity. In determining whether the Company is the primary beneficiary of an entity, the Company applies a qualitative approach that determines whether it has both (i) the power to direct the economically significant activities of the entity and (ii) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity. If the Company determines it is the primary beneficiary of a VIE, it consolidates that VIE into the Company’s financial statements. The Company’s determination about whether it should consolidate such VIEs is made continuously as changes to existing relationships or future transactions may result in a consolidation or deconsolidation event.
Property and Equipment, Net	Property and Equipment, Net Property and equipment are stated at cost, less accumulated depreciation. Depreciation is calculated using the straight-line method over the estimated useful lives of the respective assets, generally three years. Leasehold improvements are amortized on a straight-line basis over the shorter of their estimated useful lives or the term of the lease. Depreciation and amortization begins at the time the asset is placed in service. Maintenance and repairs are charged to expense as incurred and costs of improvement are capitalized.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company evaluates long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be fully recoverable. If indicators of impairment exist and the undiscounted future cash flows that the assets are expected to generate are less than the carrying value of the assets, the Company reduces the carrying amount of the assets through an impairment charge, to their estimated fair values based on a discounted cash flow approach or, when available and appropriate, to comparable market values. No impairment losses have been recorded for the periods presented.
Leases	Leases The Company leases office space and laboratory facilities under non-cancelable operating lease agreements and recognizes related rent expense on a straight-line basis over the term of the lease. Incentives granted under the Company’s facilities lease, including allowances to fund leasehold improvements and rent holidays, and are recognized as reductions to rental expense on a straight-line basis over the term of the lease. Lessor funded leasehold improvement incentives not yet received are recorded in prepaid expense and other current assets on the balance sheet. The Company does not assume renewals in its determination of the lease term unless they are deemed to be reasonably assured at the inception of the lease and begins recognizing rent expense on the date that it obtains the legal right to use and control the leased space. Deferred rent consists of the difference between cash payments and the rent expense recognized. The Company entered into capital lease agreements for certain equipment with a lease term of three years. The current portion of capital lease obligations is included in accrued and other liabilities and the noncurrent capital lease obligations is included in other noncurrent liabilities in the balance sheet.
Convertible Preferred Stock	Convertible Preferred Stock The Company records all shares of convertible preferred stock at their respective issuance price less issuance costs on the dates of issuance. Upon the occurrence of certain change in control events that are outside the Company’s control, including liquidation, sale or transfer of the Company, holders of the convertible preferred stock can cause redemption for cash. Therefore, convertible preferred stock is classified outside of stockholders’ deficit on the balance sheet as events triggering the liquidation preferences are not solely within the Company’s control. The carrying values of the convertible preferred stock are adjusted to their liquidation preferences when and if it becomes probable that such an event will occur.
Stock-Based Compensation	Stock-Based Compensation The Company measures employee and director stock-based compensation expense for all stock-based awards based on their grant date fair value. For stock-based awards with service conditions only, stock-based compensation expense is recognized over the requisite service period using the straight-line method. For awards with performance conditions, the Company evaluates the probability of achieving performance condition at each reporting date. The Company begins to recognize stock-based compensation expense using an accelerated attribution method when it is deemed probable that the performance condition will be met. Forfeitures are recognized as they occur. The Company uses the Black-Scholes option-pricing model to estimate the fair value of stock option awards that do not contain market conditions. The Black-Scholes option-pricing model requires assumptions to be made related to the expected term of an award, expected dividends, expected volatility and risk-free rate. The Company uses the Monte Carlo simulation models to estimate the fair value of stock option awards that contain market conditions. The Monte Carlo simulation models require the use of subjective and complex assumptions which determine the fair value of such awards including price volatility of the underlying stock and derived service periods. The Company recognizes stock-based compensation expense for stock options granted to non-employees based on the estimated fair value of the award as it is more readily measurable than the fair value of the services received. The fair value of stock options granted to non-employees is estimated at grant date and re-measured at each reporting period using the Black-Scholes option-pricing model until the awards vest and the resulting change in value, if any, is recognized in the statements of operations.
Income Taxes	Income Taxes The Company uses the asset and liability method of accounting for income taxes, in which deferred tax assets and liabilities are recognized for future tax consequences attributable to the differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using the enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be reversed. The effect on deferred tax assets and liabilities of a change in tax rates is recognized as income in the period that includes the enactment date. A valuation allowance is established if it is more likely than not that all or a portion of the deferred tax asset will not be realized. The Company’s tax positions are subject to income tax audits. The Company recognizes the tax benefit of an uncertain tax position only if it is more likely than not that the position is sustainable upon examination by the taxing authority, based on the technical merits. The tax benefit recognized is measured as the largest amount of benefit which is more likely than not to be realized upon settlement with the taxing authority. The Company recognizes interest accrued and penalties related to unrecognized tax benefits in its tax provision. The Company evaluates uncertain tax positions on a regular basis. The evaluations are based on a number of factors, including changes in facts and circumstances, changes in tax law, correspondence with tax authorities during the course of the audit, and effective settlement of audit issues. The provision for income taxes includes the effects of any accruals that the Company believes are appropriate, as well as the related net interest and penalties.
Net Loss per Common Share	Net Loss per Common Share Basic net loss per share is calculated by dividing net loss by the weighted average number of shares outstanding for the period. Diluted net loss per share is calculated by dividing net loss by the weighted average number of shares of common stock and potential dilutive common stock equivalents outstanding during the period if the effect is dilutive. The calculation of diluted earnings (loss) per share also requires that, to the extent the presumed issuance of additional shares as contingent consideration is dilutive to earnings (loss) per share for the period, adjustments to net income or net loss used in the calculation are required to remove the change in fair value of the contingent consideration liability for the period. Likewise, adjustments to the denominator are required to reflect the related dilutive shares. In all periods presented, the Company’s outstanding stock options, convertible preferred stock, early exercised common stock subject to future vesting, restricted stock accounted for as options common and preferred stock warrants and presumed issuance of additional shares as contingent consideration were excluded from the calculation of diluted net loss per share because their effects were antidilutive.
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss and certain changes in stockholders’ deficit that are excluded from net loss, primarily unrealized losses on the Company’s marketable securities.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In August 2018, the FASB issued ASU No. 2018-15, Intangibles (Topic 350): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. This new standard also requires customers to expense the capitalized implementation costs of a hosting arrangement that is a service contract over the term of the hosting arrangement. This standard is effective for the Company for annual reporting periods beginning after December 15, 2020, and interim periods within annual periods beginning after December 15, 2021. This new standard can be applied either retrospectively or prospectively to all implementation costs incurred after the date of adoption. The Company is currently evaluating the impact of adoption on its financial statements. In August 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the SEC adopted amendments to certain disclosure requirements in Securities Act Release No. 33-10532, Disclosure Update and Simplification. The Company adopted these SEC amendments on November 5, 2018 and will present the analysis of changes in stockholders’ equity in its interim financial statements in its March 31, 2019 Form 10-Q. The Company does not anticipate that the adoption of these SEC amendments will have a material effect on the Company’s financial position, results of operations, cash flows or shareholders’ equity. In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting In January 2017, the FASB issued ASU No. 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230: Classification of Certain Cash Receipts and Cash Payments) In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities In May 2017, the FASB issued ASU No. 2017-09, Compensation-Stock Compensation (Topic 718)- Scope of Modification Accounting
Fair Value Measurements	Fair Value Measurements The Company determines the fair value of financial and non-financial assets and liabilities based on the assumptions that market participants would use in pricing the asset or liability in an orderly transaction between market participants at the measurement date. The identification of market participant assumptions provides a basis for determining what inputs are to be used for pricing each asset or liability. A fair value hierarchy has been established which gives precedence to fair value measurements calculated using observable inputs over those using unobservable inputs. This hierarchy prioritized the inputs into three broad levels as follows: • Level 1: Quoted prices in active markets for identical instruments • Level 2: Other significant observable inputs (including quoted prices in active markets for similar instruments) • Level 3: Significant unobservable inputs (including assumptions in determining the fair value of certain investments) The carrying amounts of financial instruments such as cash and cash equivalents, restricted cash, contribution receivable, prepaid expenses and other current assets, accounts payable, accrued compensation, accrued and other current liabilities approximate the related fair values due to the short maturities of these instruments.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets that sum to the total of the same amounts shown in the statements of cash flows. December 31, 2018 2017 2016 (in thousands) Cash and cash equivalents $ 15,399 $ 7,298 $ 89,286 Restricted cash 550 550 450 Total cash, cash equivalents, and restricted cash $ 15,949 $ 7,848 $ 89,736

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Summary of Financial Assets Subject to Fair Value Measurements on Recurring Basis and Level of Inputs Used in Measurements	The Company’s financial assets subject to fair value measurements on a recurring basis and the level of inputs used in such measurements were as follows (in thousands): December 31, 2018 Total Level 1 Level 2 Level 3 Assets: Cash equivalents: Money market funds $ 14,131 $ 14,131 $ — $ — Total cash equivalents 14,131 14,131 — — Short-term marketable securities: U.S. treasuries 34,121 — 34,121 — U.S. and foreign commercial paper 10,635 — 10,635 — U.S. and foreign corporate debt securities 26,533 — 26,533 — Asset-backed securities 2,748 — 2,748 — U.S. government debt securities 81,699 — 81,699 — Total short-term marketable securities 155,736 — 155,736 — Total assets subject to fair value measurements on a recurring basis $ 169,867 $ 14,131 $ 155,736 $ — Liabilities: Contingent consideration liability $ 2,483 $ — $ — $ 2,483 Total liabilities subject to fair value measurements on a recurring basis $ 2,483 $ — $ — $ 2,483 December 31, 2017 Total Level 1 Level 2 Level 3 Assets: Cash equivalents: Money market funds $ 5,709 $ 5,709 $ — $ — Total cash equivalents 5,709 5,709 — — Short-term marketable securities: U.S. and foreign commercial paper 6,359 — 6,359 — U.S. and foreign corporate debt securities 16,149 — 16,149 — Asset-backed securities 14,588 — 14,588 — U.S. government debt securities 40,362 — 40,362 — U.S. treasuries 1,754 — 1,754 — Total short-term marketable securities 79,212 — 79,212 — Long-term marketable securities: Asset-backed securities 2,742 — 2,742 — U.S. government debt securities 2,376 — 2,376 — Total long-term marketable securities 5,118 — 5,118 — Total marketable securities 84,330 — 84,330 — Total assets subject to fair value measurements on a recurring basis $ 90,039 $ 5,709 $ 84,330 $ —
Summary of Reconciliation of Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs	As of December 31, 2018, the Company determined that the net settlement criteria of the definition of a derivative had been met for 133,333 shares of common stock to the third parties. The Company issued 106,666 of these shares in January 2019 and will issue the remaining 26,667 shares in early 2019 and recorded a settlement liability of $2.0 million at December 31, 2018. The Company recorded a contingent consideration liability of $2.5 million at December 31, 2018 related to additional potential shares subject to the achievement of certain specified clinical development and sales milestone events under the agreements. No liability was recorded at December 31, 2017 as the net settlement criteria were not met. The following table provides a reconciliation of assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs (Level 3) (in thousands): Amount Balance at December 31, 2017 $ — Additions — Settlements — Change in fair value 4,542 Balance at December 31, 2018 $ 4,542

Marketable Securities (Tables)

Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Summary of Marketable Securities Classified as Available-for-Sale	Marketable securities, which are classified as available-for-sale, consisted of the following (in thousands): December 31, 2018 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value Cash equivalents: Money market funds $ 14,131 $ — $ — $ 14,131 Total cash equivalents 14,131 — — 14,131 Short-term marketable securities: U.S. and foreign commercial paper 10,638 — (3 ) 10,635 U.S. and foreign corporate debt securities 26,552 2 (21 ) 26,533 Asset-backed securities 2,750 — (2 ) 2,748 U.S. government debt securities 81,755 1 (57 ) 81,699 U.S. treasuries 34,136 1 (16 ) 34,121 Total short-term marketable securities 155,831 4 (99 ) 155,736 Total marketable securities $ 169,962 $ 4 $ (99 ) $ 169,867 December 31, 2017 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value Cash equivalents: Money market funds $ 5,709 $ — $ — $ 5,709 Total cash equivalents 5,709 — — 5,709 Short-term marketable securities: U.S. and foreign commercial paper 6,369 — (10 ) 6,359 U.S. and foreign corporate debt securities 16,162 — (13 ) 16,149 Asset-backed securities 14,604 — (16 ) 14,588 U.S. government debt securities 40,418 — (56 ) 40,362 U.S. treasuries 1,754 — — 1,754 Total short-term marketable securities 79,307 — (95 ) 79,212 Long-term marketable securities: Asset-backed securities 2,752 — (10 ) 2,742 U.S. government debt securities 2,375 1 — 2,376 Total long-term marketable securities 5,127 1 (10 ) 5,118 Total marketable securities $ 90,143 $ 1 $ (105 ) $ 90,039

Balance Sheet Components (Tabl

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net, consists of the following: December 31, 2018 2017 (in thousands) Laboratory equipment $ 4,162 $ 2,614 Computer equipment 247 137 Furniture and fixtures 113 105 Leasehold improvements 5,366 5,346 Construction in progress — 226 Total property and equipment 9,888 8,428 Less: accumulated depreciation and amortization (3,650 ) (1,470 ) Total property and equipment, net $ 6,238 $ 6,958
Schedule of Accrued and Other Current Liabilities	Accrued and other current liabilities consist of the following: December 31, 2018 2017 (in thousands) Accrued research and development $ 1,837 $ 2,105 Deferred rent, current portion 783 702 Professional fees 26 70 Liability related to early exercise shares 885 257 Accrued other 1,459 204 $ 4,990 $ 3,338

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Summary of Future Minimum Lease Payments under Non-cancellable Operating Lease	As of December 31, 2018, the Company’s future minimum payments under the noncancelable operating lease is as follows (in thousands): Amount 2019 $ 2,012 2020 2,072 2021 2,135 2022 1,621 Total future minimum lease payments $ 7,840

Convertible Preferred Stock a_2

Convertible Preferred Stock and Common Stock (Tables)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Summary of Convertible Preferred Stock	As of December 31, 2017, convertible preferred stock consisted of the following (in thousands, except share amounts): Shares Authorized Shares Issued and Outstanding Liquidation Preference Carrying Value Series A-1 9,085,738 2,887,086 $ 2,495 $ 2,457 Series A-2 32,653,411 10,498,269 9,198 9,214 Series B 50,000,000 14,774,369 179,132 162,285 Total convertible preferred stock 91,739,149 28,159,724 $ 190,825 $ 173,956

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Summary of Stock Option Activity	A summary of the Company’s stock option activity under the 2013 and 2018 Plan is as follows: Shares Available for Grant Outstanding Options Weighted- Average Exercise Price Weighted- Average Remaining Contract Term Aggregate Intrinsic Value (in Years) (in thousands) Balances at December 31, 2017 918,595 4,196,213 $ 3.06 Retired from 2013 Plan (378,875 ) — — Authorized 4,289,936 — — Granted (2,082,265 ) 2,082,265 13.20 Exercised — (501,329 ) 3.17 Canceled 280,411 (276,618 ) 5.82 Balances at December 31, 2018 3,027,802 5,500,531 $ 6.75 8.49 $ 53,051 Vested and exercisable at December 31, 2018 1,619,419 $ 2.72 7.65 $ 21,929 Vested and expected to vest at December 31, 2018 5,500,531 $ 6.75 8.49 $ 53,051
Summary of Valuation Assumption to Estimate Fair Value of Stock Options	The fair value of stock options granted to employees was estimated on the date of grant using the Black-Scholes option pricing model using the following assumptions: Year Ended December 31, 2018 2017 2016 Expected dividend yield — — — Expected term of options (in years) 6.1 5.6–6.7 5.3–6.1 Risk-free interest rate 2.6%-3.0% 1.8%–2.2% 1.2%–2.1% Expected stock price volatility 87.4%-92.6% 77.0%–82.0% 76.1%–79.7%
Summary of Restricted Stock Activity	A summary of the Company’s restricted stock activity for the year ended December 31, 2018 was as follows: Shares Weighted- Average Grant Date Fair Value Unvested at December 31, 2017 478,971 $ 4.57 Granted — $ 4.57 Vested (119,742 ) $ 4.57 Unvested at December 31, 2018 359,229 $ 4.57
Summary of Stock-based Compensation Expense	The following table sets forth the total stock-based compensation expense for all options granted to employees and nonemployees, including shares sold through the issuance of non-recourse promissory notes which are considered to be options for accounting purposes, and costs associated with the Company’s 2018 ESPP included in the Company’s statement of operations (in thousands): Year Ended December 31, 2018 2017 2016 Research and development $ 6,043 $ 1,695 $ 164 General and administrative 3,398 1,339 60 Total $ 9,441 $ 3,034 $ 224
2018 Employee Stock Purchase Plan
Summary of Valuation Assumptions to Estimate Fair Values of Rights Granted	The fair values of the rights granted under the 2018 ESPP were calculated using the following assumptions: Year Ended December 31, 2018 Expected dividend yield — Expected term of options (in years) 0.7 Risk-free interest rate 2.41% Expected stock price volatility 73.18%

Net Loss per Common Share (Tabl

Net Loss per Common Share (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Schedule of Reconciliation of Numerators and Denominators Used In Computing Net Loss From Continuing Operations Per Share	A reconciliation of the numerators and denominators used in computing net loss from continuing operations per share is as follows (in thousands, except per share amounts): December 31, 2018 2017 2016 (in thousands, except share and per share amounts) Numerator: Net loss $ (76,398 ) $ (44,656 ) $ (30,404 ) Denominator: Weighted average number of shares outstanding—basic and diluted 28,269,907 3,197,516 2,662,841 Net loss per share—basic and diluted $ (2.70 ) $ (13.97 ) $ (11.42 )
Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Per Share	Potentially dilutive securities that were not included in the diluted per share calculations because they would be anti-dilutive were as follows: Year Ended December 31, 2018 2017 2016 Convertible preferred stock — 28,159,724 24,620,615 Options to purchase common stock 5,500,531 4,365,694 508,418 Early exercised common stock subject to future vesting 704,028 831,439 1,287,435 Restricted stock accounted for as options 359,228 625,084 — Warrants to purchase convertible preferred stock — 763,501 763,501 Warrants to purchase common stock — 96,610 96,610 Shares subject to the 2018 ESPP 27,622 — — Total 6,591,409 34,842,052 27,276,579

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Effective Tax Rate of Provision for Income Taxes Differs From Federal Statutory Rate	The effective tax rate of the provision for income taxes differs from the federal statutory rate as follows: Year Ended December 31, 2018 2017 2016 Taxes at the U.S. statutory income tax rate 21.0 % 34.0 % 34.0 % State tax, net of federal benefit 0.9 — — Other (0.1 ) (2.2 ) — Stock-based compensation 0.3 — — General business credits 1.0 — — Change in valuation allowance (23.1 ) (13.3 ) (21.0 ) Non-deductible interest expense — — (13.0 ) Change in income tax rate due to Tax Act — (18.5 ) — Total provision for income taxes — % — % — %
Schedule of Components of Deferred Tax Assets	The tax effects of significant items comprising the Company’s deferred tax assets are as follows: December 31, 2018 2017 (in thousands) Deferred tax assets: Net operating loss $ 29,926 $ 16,530 Research and development credits 3,865 1,879 Stock-based compensation 1,839 671 Contingent consideration 954 — Accruals and other 1,040 895 Charitable contributions 253 330 Total deferred tax assets 37,877 20,305 Valuation allowance (37,877 ) (20,236 ) Net deferred tax assets — 69 Deferred tax liability — (69 ) Net deferred tax assets $ — $ —
Summary of Net Operating Losses and Tax Credit Carryforwards	Net operating losses and tax credit carryforwards as of December 31, 2018 are as follows: Amount Expiration Years Net operating losses, federal (post December 31, 2017) $ 64,593 indefinite Net operating losses, federal (pre January 1, 2018) 64,136 2030 - 2038 Net operating losses, state 64,663 2030 - 2039 Tax credits, federal 2,988 2031 - 2039 Tax credits, state 2,692 Indefinite
Schedule of Reconciliation of Unrecognized Tax Benefits	A reconciliation of the beginning and ending amounts of unrecognized tax benefits is as follows: December 31, 2018 2017 (in thousands) Gross unrecognized tax benefits at January 1 $ 3,065 $ 2,800 Additions for tax positions taken in the current year 753 478 Reductions for tax positions taken in the prior year (104 ) (213 ) Gross unrecognized tax benefits at December 31 $ 3,714 $ 3,065

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information	The following tables show a summary of the Company’s quarterly financial information for each of the four quarters of 2018 and 2017 and has been prepared in accordance with GAAP for interim financial reporting (in thousands, except for per share data): Quarter Year Ended December 31, 2018 First Second Third Fourth Loss from operations $ (16,482 ) $ (20,798 ) $ (19,377 ) $ (22,808 ) Net loss $ (16,133 ) $ (20,002 ) $ (18,346 ) $ (21,917 ) Net loss per common share, basic and diluted $ (4.69 ) $ (0.76 ) $ (0.45 ) $ (0.53 ) Quarter Year Ended December 31, 2017 First Second Third Fourth Loss from operations $ (9,040 ) $ (11,698 ) $ (12,083 ) $ (12,787 ) Net loss $ (8,934 ) $ (11,428 ) $ (11,750 ) $ (12,544 ) Net loss per common share, basic and diluted $ (2.90 ) $ (3.62 ) $ (3.63 ) $ (3.67 )

Organization - Additional Infor

Organization - Additional Information (Details) - USD ($) $ 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
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Net loss	$ (21,917)	$ (18,346)	$ (20,002)	$ (16,133)	$ (12,544)	$ (11,750)	$ (11,428)	$ (8,934)	$ (76,398)	$ (44,656)	$ (30,404)
Cash used in operations									(56,623)	(38,358)	$ (16,398)
Accumulated deficit	$ (163,278)				$ (86,880)				$ (163,278)	$ (86,880)

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details)	May 07, 2018USD ($)$ / sharesshares	Apr. 20, 2018	Dec. 31, 2018USD ($)Segment$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016shares	Mar. 31, 2018shares	Feb. 28, 2018shares	Jul. 30, 2017USD ($)
Summary Of Significant Accounting Policies [Line Items]
Reverse split of shares of common and convertible preferred stock, conversion ratio		0.33898
Reverse split of shares of common and convertible preferred stock, effective date		Apr. 20, 2018
Net proceeds from issuance initial public offering after Deducting Underwriting Discounts Commissions and offering related transaction costs | $			$ 79,055,000
Common stock, shares authorized			300,000,000	122,000,000		140,000,000	122,000,000
Common stock, par value | $ / shares			$ 0.0001	$ 0.0001
Preferred stock, shares authorized						103,283,818	91,739,149
Number of segments | Segment			1
Off-balance sheet concentrations of credit risk description			As of December 31, 2018, the Company had no off-balance sheet concentrations of credit risk.
Off-balance sheet concentrations of credit risk | $			$ 0
Recognition of revenue | $				$ 1,382,000
Maximum
Summary Of Significant Accounting Policies [Line Items]
Funding provided for the research and development activities | $								$ 1,500,000
Common Stock
Summary Of Significant Accounting Policies [Line Items]
Issuance of common stock (in shares)			5,000,000	12,711	736,161
Common shares issued upon conversion of preferred shares			32,073,149
IPO
Summary Of Significant Accounting Policies [Line Items]
Issuance of common stock (in shares)	5,000,000
Issuance price per share | $ / shares	$ 17
Net proceeds from issuance initial public offering after Deducting Underwriting Discounts Commissions and offering related transaction costs | $	$ 75,900,000
Approximate underwriting discounts commissions and offering related transaction costs | $	$ 9,100,000
Common stock, shares authorized	300,000,000
Common stock, par value | $ / shares	$ 0.0001
Preferred stock, shares authorized	10,000,000
Preferred stock, par value | $ / shares	$ 0.0001
IPO | Common Stock
Summary Of Significant Accounting Policies [Line Items]
Common shares issued upon conversion of preferred shares	32,073,149
IPO | Common Stock | Minimum
Summary Of Significant Accounting Policies [Line Items]
Net proceeds from issuance initial public offering after Deducting Underwriting Discounts Commissions and offering related transaction costs | $			$ 30,000,000
Amended and Restated Certificate of Incorporation
Summary Of Significant Accounting Policies [Line Items]
Reverse split of shares of common and convertible preferred stock		1-for-2.95 reverse split

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Cash and cash equivalents	$ 15,399	$ 7,298	$ 89,286
Restricted cash	550	550	450
Total cash, cash equivalents, and restricted cash	$ 15,949	$ 7,848	$ 89,736	$ 940

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets Subject to Fair Value Measurements on Recurring Basis and Level of Inputs Used in Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
Short-term marketable securities	$ 155,736	$ 79,212
Long-term marketable securities		5,118
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	14,131	5,709
Short-term marketable securities	155,736	79,212
Long-term marketable securities		5,118
Total marketable securities		84,330
Total assets subject to fair value measurements on a recurring basis	169,867	90,039
Liabilities:
Total liabilities subject to fair value measurements on a recurring basis	2,483
Fair Value, Measurements, Recurring | Level 1
Assets:
Cash equivalents	14,131	5,709
Total assets subject to fair value measurements on a recurring basis	14,131	5,709
Fair Value, Measurements, Recurring | Level 2
Assets:
Short-term marketable securities	155,736	79,212
Long-term marketable securities		5,118
Total marketable securities		84,330
Total assets subject to fair value measurements on a recurring basis	155,736	84,330
Fair Value, Measurements, Recurring | Level 3
Liabilities:
Total liabilities subject to fair value measurements on a recurring basis	2,483
Fair Value, Measurements, Recurring | Contingent Consideration Liability
Liabilities:
Total liabilities subject to fair value measurements on a recurring basis	2,483
Fair Value, Measurements, Recurring | Contingent Consideration Liability | Level 3
Liabilities:
Total liabilities subject to fair value measurements on a recurring basis	2,483
Money market funds | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	14,131	5,709
Money market funds | Fair Value, Measurements, Recurring | Level 1
Assets:
Cash equivalents	14,131	5,709
U.S. Treasuries | Fair Value, Measurements, Recurring
Assets:
Short-term marketable securities	34,121	1,754
U.S. Treasuries | Fair Value, Measurements, Recurring | Level 2
Assets:
Short-term marketable securities	34,121	1,754
U.S. and Foreign Commercial Paper | Fair Value, Measurements, Recurring
Assets:
Short-term marketable securities	10,635	6,359
U.S. and Foreign Commercial Paper | Fair Value, Measurements, Recurring | Level 2
Assets:
Short-term marketable securities	10,635	6,359
Corporate Debt Securities | Fair Value, Measurements, Recurring
Assets:
Short-term marketable securities	26,533	16,149
Corporate Debt Securities | Fair Value, Measurements, Recurring | Level 2
Assets:
Short-term marketable securities	26,533	16,149
Asset-backed Securities | Fair Value, Measurements, Recurring
Assets:
Short-term marketable securities	2,748	14,588
Long-term marketable securities		2,742
Asset-backed Securities | Fair Value, Measurements, Recurring | Level 2
Assets:
Short-term marketable securities	2,748	14,588
Long-term marketable securities		2,742
US Government Debt Securities | Fair Value, Measurements, Recurring
Assets:
Short-term marketable securities	81,699	40,362
Long-term marketable securities		2,376
US Government Debt Securities | Fair Value, Measurements, Recurring | Level 2
Assets:
Short-term marketable securities	$ 81,699	40,362
Long-term marketable securities		$ 2,376

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Settlement liability			$ 2,000,000
Fair value assets hierarchy levels transfers amount			0	$ 0
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities fair value disclosure			2,500,000	$ 0
Commercial Agreements | Contingent Consideration | Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Maximum amount of potential contingent consideration value			$ 8,700,000
Commercial Agreements | Clinical Stage Biopharmaceutical Company | Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent consideration additional common stock issued			666,670
Compound Library And Option Agreement
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Common shares expected to be issued			133,333
Compound Library And Option Agreement | Scenario, Forecast | Academic Institution
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Common shares expected to be issued		26,667
Compound Library And Option Agreement | Subsequent Event | Licensor
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Issuance of common stock (in shares)	106,666

Fair Value Measurements - Sum_2

Fair Value Measurements - Summary of Reconciliation of Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Details) - Fair Value, Measurements, Recurring - Level 3 $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Balance at December 31, 2017	$ 0
Additions	0
Settlements	0
Change in fair value	4,542
Balance at December 31, 2018	$ 4,542

Marketable Securities - Summary

Marketable Securities - Summary of Marketable Securities Classified as Available-for-Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	$ 169,962	$ 90,143
Unrealized Gains	4	1
Unrealized Losses	(99)	(105)
Fair Value	169,867	90,039
Cash Equivalents
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	14,131	5,709
Fair Value	14,131	5,709
Cash Equivalents | Money market funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	14,131	5,709
Fair Value	14,131	5,709
Short-Term Marketable Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	155,831	79,307
Unrealized Gains	4
Unrealized Losses	(99)	(95)
Fair Value	155,736	79,212
Short-Term Marketable Securities | U.S. and Foreign Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	10,638	6,369
Unrealized Losses	(3)	(10)
Fair Value	10,635	6,359
Short-Term Marketable Securities | Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	26,552	16,162
Unrealized Gains	2
Unrealized Losses	(21)	(13)
Fair Value	26,533	16,149
Short-Term Marketable Securities | Asset-backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	2,750	14,604
Unrealized Losses	(2)	(16)
Fair Value	2,748	14,588
Short-Term Marketable Securities | US Government Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	81,755	40,418
Unrealized Gains	1
Unrealized Losses	(57)	(56)
Fair Value	81,699	40,362
Short-Term Marketable Securities | U S Treasuries
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	34,136	1,754
Unrealized Gains	1
Unrealized Losses	(16)
Fair Value	$ 34,121	1,754
Long-Term Marketable Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis		5,127
Unrealized Gains		1
Unrealized Losses		(10)
Fair Value		5,118
Long-Term Marketable Securities | Asset-backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis		2,752
Unrealized Losses		(10)
Fair Value		2,742
Long-Term Marketable Securities | US Government Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis		2,375
Unrealized Gains		1
Fair Value		$ 2,376

Marketable Securities - Additio

Marketable Securities - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Investments Debt And Equity Securities [Abstract]
Realized gains or losses on available-for-sale securities	$ 0

License Agreements - Additional

License Agreements - Additional Information (Details)	1 Months Ended		12 Months Ended
	Jan. 31, 2019shares	Apr. 30, 2016USD ($)	Dec. 31, 2019shares	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)Milestoneshares	Dec. 31, 2016USD ($)	May 31, 2018USD ($)	Jul. 30, 2017USD ($)	Feb. 29, 2016shares
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common stock, shares issued | shares				42,414,294	4,830,389
Settlement liability				$ 2,059,000
Research and development expense				58,907,000	$ 37,373,000	$ 13,707,000
Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Funding provided for the research and development activities								$ 1,500,000
Clinical Stage Biopharmaceutical Company
Research And Development Arrangement Contract To Perform For Others [Line Items]
Number of milestones achieved | Milestone					0
Clinical Stage Biopharmaceutical Company | Licensed Products
Research And Development Arrangement Contract To Perform For Others [Line Items]
Royalties due from sales				$ 0	$ 0
Commercial Agreements
Research And Development Arrangement Contract To Perform For Others [Line Items]
Funding provided for the research and development under the agreement in period				2020-02
Research and development expense				$ 500,000	500,000
Commercial Agreements | Clinical Stage Biopharmaceutical Company
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common stock, shares issued | shares									533,335
Commercial Agreements | Clinical Stage Biopharmaceutical Company | Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Milestone payments				$ 70,300,000
Contingent consideration additional common stock issued | shares				666,670
Funding provided for the research and development activities				$ 2,000,000
Commercial Agreements | Academic Institution
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common stock, shares issued | shares									133,333
Commercial Agreements | Affiliate of Clinical-Stage Biopharmaceutical Company
Research And Development Arrangement Contract To Perform For Others [Line Items]
Aggregate purchase price of equity interest		$ 500,000
Additional amount invested							$ 500,000
Compound Library And Option Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common shares expected to be issued | shares				133,333
Settlement liability				$ 2,000,000
Contingent consideration liability				2,500,000
Compound Library And Option Agreement | Scenario, Forecast | Academic Institution
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common shares expected to be issued | shares			26,667
Compound Library And Option Agreement | Subsequent Event | Licensor
Research And Development Arrangement Contract To Perform For Others [Line Items]
Issuance of common stock (in shares) | shares	106,666
Other Licensing Agreements with Research Institutions
Research And Development Arrangement Contract To Perform For Others [Line Items]
Contingent consideration, milestone or royalty payments				$ 0	$ 0

Balance Sheet Components - Sche

Balance Sheet Components - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Total property and equipment	$ 9,888	$ 8,428
Less: accumulated depreciation and amortization	(3,650)	(1,470)
Total property and equipment, net	6,238	6,958
Laboratory Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	4,162	2,614
Computer Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	247	137
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	113	105
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	$ 5,366	5,346
Construction in progress
Property Plant And Equipment [Line Items]
Total property and equipment		$ 226

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Depreciation expense	$ 2.2	$ 1.3	$ 0.2

Balance Sheet Components - Sc_2

Balance Sheet Components - Schedule of Accrued and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued research and development	$ 1,837	$ 2,105
Deferred rent, current portion	783	702
Professional fees	26	70
Liability related to early exercise shares	885	257
Accrued other	1,459	204
Accrued and other current liabilities	$ 4,990	$ 3,338

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Commitments And Contingencies [Line Items]
Operating lease, expiration date	Oct. 31, 2022
Operating lease, renewal term	4 years
Operating leases rent holiday period for expanded space	3 months
Tenant improvements allowance	$ 3.9
Rent expense, operating lease	$ 1.8	$ 2	$ 0.8
Minimum
Commitments And Contingencies [Line Items]
Operating lease option exercise notice period	15 months

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Future Minimum Lease Payments under Non-cancellable Operating Lease (Details) $ in Thousands	Dec. 31, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,019	$ 2,012
2,020	2,072
2,021	2,135
2,022	1,621
Total future minimum lease payments	$ 7,840

Related-Party Transactions - Ad

Related-Party Transactions - Additional Information (Details) $ in Thousands	Apr. 04, 2018USD ($)	Apr. 30, 2018USD ($)	Jan. 31, 2018USD ($)shares	Oct. 31, 2017USD ($)Promissoryshares	Jul. 31, 2016USD ($)shares	Apr. 30, 2016USD ($)shares	Dec. 31, 2015USD ($)shares	Dec. 31, 2018USD ($)PromissoryNoteExecutive	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Related Party Transaction [Line Items]
Number of promissory notes | PromissoryNote								3
Number of executive officers | Executive								2
Proceeds from issuance of convertible preferred stock								$ 59,881	$ 42,867	$ 90,956
Research and development expense								58,907	37,373	13,707
Master Services Agreement
Related Party Transaction [Line Items]
Research and development expense								400	600
Executive Officer
Related Party Transaction [Line Items]
Due from related parties				$ 2,100
Restricted stock purchased by related party, shares | shares				625,084
Forgiveness of promissory note accounted as modification of share based payment								1,500
Stockholders | Financing Activities
Related Party Transaction [Line Items]
Proceeds from issuance of convertible preferred stock								$ 3,000
Stockholders | Series B Preferred Stock | Financing Activities
Related Party Transaction [Line Items]
Proceeds from issuance of convertible preferred stock									$ 8,000
Stockholders and Executive Officers | Series B Preferred Stock | Financing Activities
Related Party Transaction [Line Items]
Proceeds from issuance of convertible preferred stock										$ 32,800
Promissory Note | Executive Officer
Related Party Transaction [Line Items]
Number of promissory notes | Promissory				2
Due from related parties			$ 400		$ 200	$ 200	$ 200
Interest rate			2.50%		2.50%	2.50%	2.50%
Promissory Note | Common Stock | Executive Officer
Related Party Transaction [Line Items]
Early exercise options in aggregate | shares			114,406		667,253	667,253	667,253
Promissory Note One | Executive Officer
Related Party Transaction [Line Items]
Due from related parties				$ 1,600
Interest rate				1.85%
Promissory Note Two | Executive Officer
Related Party Transaction [Line Items]
Due from related parties				$ 500
Interest rate				1.85%
Non-Recourse Promissory Note | Executive Officer
Related Party Transaction [Line Items]
Outstanding principal and accrued interest, forgiveness		$ 1,600
Repayment of promissory notes	$ 500
Full-recourse Notes | Executive Officer
Related Party Transaction [Line Items]
Repayment of promissory notes	$ 200

Convertible Preferred Stock a_3

Convertible Preferred Stock and Common Stock - Additional Information (Details)	May 07, 2018USD ($)shares	Apr. 30, 2018USD ($)$ / sharesshares	Mar. 31, 2018USD ($)$ / sharesshares	Jun. 30, 2017USD ($)$ / sharesshares	Mar. 31, 2017USD ($)$ / sharesshares	Oct. 31, 2016USD ($)$ / sharesshares	Feb. 29, 2016USD ($)$ / sharesshares	Jul. 31, 2013USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)	Mar. 31, 2015USD ($)$ / sharesshares	Feb. 28, 2018shares
Class Of Stock [Line Items]
Number of stock, shares issued									0	28,159,724
Common stock, shares authorized			140,000,000						300,000,000	122,000,000			122,000,000
Preferred stock, shares authorized			103,283,818										91,739,149
Net proceeds from sale of convertible preferred stock | $									$ 59,881,000	$ 42,867,000	$ 90,956,000
Gross proceeds from public offering | $									$ 79,055,000
Convertible preferred stock warrants converted into warrants	763,501
Number of stock, shares outstanding									0	28,159,724
IPO
Class Of Stock [Line Items]
Common stock, shares authorized	300,000,000
Preferred stock, shares authorized	10,000,000
Gross proceeds from public offering | $	$ 75,900,000
Common Stock
Class Of Stock [Line Items]
Conversion ratio of preferred to common shares									1
Common shares issued upon conversion of preferred shares									32,073,149
Common Stock | IPO
Class Of Stock [Line Items]
Common shares issued upon conversion of preferred shares	32,073,149
Minimum percentage of preferred shareholders required to request conversion									60.00%
Common Stock | Minimum | IPO
Class Of Stock [Line Items]
Gross proceeds from public offering | $									$ 30,000,000
Share price | $ / shares									$ 15.3317
Series A-1 Convertible Preferred Stock
Class Of Stock [Line Items]
Number of stock, shares issued								2,887,086		2,887,086
Shares price per share | $ / shares								$ 0.864
Gross proceeds from issuance of convertible preferred stock | $								$ 2,000,000
Number of stock, shares outstanding										2,887,086
Conversion price of preferred stock | $ / shares										$ 0.864
Liquidation preference per share | $ / shares										$ 0.864
Series A-2 Convertible Preferred Stock
Class Of Stock [Line Items]
Number of stock, shares issued							4,671,430			10,498,269		5,826,839
Shares price per share | $ / shares							$ 0.876					$ 0.876
Gross proceeds from issuance of convertible preferred stock | $							$ 4,000,000					$ 4,900,000
Number of stock, shares outstanding										10,498,269
Conversion price of preferred stock | $ / shares										$ 0.876
Liquidation preference per share | $ / shares										$ 0.876
Series B Convertible Preferred Stock
Class Of Stock [Line Items]
Number of stock, shares issued				2,879,288	659,821	7,519,592				14,774,369
Shares price per share | $ / shares				$ 12.125	$ 12.125	$ 12.125
Gross proceeds from issuance of convertible preferred stock | $				$ 34,900,000	$ 8,000,000	$ 91,200,000
Additional number of stock, Shares issued, Amount | $						$ 9,000,000
Forward options expirey period						180 days
Number of stock, shares outstanding										14,774,369
Conversion price of preferred stock | $ / shares										$ 12.125
Liquidation preference per share | $ / shares										$ 12.125
Series C Convertible Preferred Stock
Class Of Stock [Line Items]
Number of stock, shares issued		322,852	3,590,573
Shares price per share | $ / shares		$ 15.3317	$ 15.3317
Preferred stock, shares designated			11,544,669
Net proceeds from sale of convertible preferred stock | $		$ 5,000,000	$ 54,900,000
Liquidation preference per share | $ / shares		$ 15.3317
Series C Convertible Preferred Stock | U.S. National Securities Exchange | Minimum
Class Of Stock [Line Items]
Gross proceeds from public offering | $			$ 30,000,000
Series C Convertible Preferred Stock | Common Stock
Class Of Stock [Line Items]
Conversion ratio of preferred to common shares			1
Noncumulative Preferred Stock
Class Of Stock [Line Items]
Dividends rate of respective series of convertible preferred stock issuance price									6.00%

Convertible Preferred Stock a_4

Convertible Preferred Stock and Common Stock - Summary of Convertible Preferred Stock (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Oct. 31, 2016	Feb. 29, 2016	Mar. 31, 2015	Jul. 31, 2013
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized	10,000,000	91,739,149
Convertible preferred stock, Shares Issued	0	28,159,724
Convertible preferred stock, Shares Outstanding	0	28,159,724
Convertible preferred stock, Liquidation Preference	$ 0	$ 190,825
Convertible preferred stock, Carrying Value		$ 173,956
Series A-1
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized		9,085,738
Convertible preferred stock, Shares Issued		2,887,086						2,887,086
Convertible preferred stock, Shares Outstanding		2,887,086
Convertible preferred stock, Liquidation Preference		$ 2,495
Convertible preferred stock, Carrying Value		$ 2,457
Series A-2
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized		32,653,411
Convertible preferred stock, Shares Issued		10,498,269				4,671,430	5,826,839
Convertible preferred stock, Shares Outstanding		10,498,269
Convertible preferred stock, Liquidation Preference		$ 9,198
Convertible preferred stock, Carrying Value		$ 9,214
Series B
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized		50,000,000
Convertible preferred stock, Shares Issued		14,774,369	2,879,288	659,821	7,519,592
Convertible preferred stock, Shares Outstanding		14,774,369
Convertible preferred stock, Liquidation Preference		$ 179,132
Convertible preferred stock, Carrying Value		$ 162,285

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details)	1 Months Ended			6 Months Ended	12 Months Ended
	Mar. 31, 2018USD ($)shares	Oct. 31, 2017Agreementshares	Jun. 30, 2013	Feb. 28, 2018$ / shares	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)$ / shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares of common stock authorized for issuance					4,289,936
Intrinsic value of options exercised | $					$ 1,500,000	$ 100,000	$ 20,000
Weighted-average estimated fair value of stock options granted | $ / shares					$ 13.20	$ 3.40	$ 0.32
Exercisable, aggregate intrinsic value | $					$ 21,929,000	$ 3,300,000
Stock-based compensation cost not yet amortized | $					$ 20,600,000
Weighted-average period for recognition					4 years 1 month 6 days
Total grant-date fair value of stock options vested | $					$ 3,500,000	$ 1,500,000
Stock option awards outstanding					5,500,531	4,196,213
Stock option awards, grant date fair value | $					$ 53,051,000
Options granted in period to purchase an aggregate of shares					2,082,265
Weighted-Average Exercise Price, Options granted | $ / shares					$ 13.20
Stock-based compensation expense | $					$ 9,441,000	$ 3,034,000	$ 224,000
2018 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance	536,242
Option purchase date price percentage lower of closing trading price per share	85.00%
Employees subscription rate during the offering period	15.00%
Maximum number of shares may be purchased by an employee	15,000
Maximum fair market value of shares may be purchased by an employee | $	$ 25,000
Non Employee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted in period to purchase an aggregate of shares					20,337	235,250
Aggregate of shares issued outside the plan						169,491
Weighted-Average Exercise Price, Options granted | $ / shares					$ 6.19	$ 3.39
Stock-based compensation expense | $					$ 1,200,000	$ 400,000
Performance Contingent Stock Options Granted to Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Estimated fair value of stock options granted | $					$ 400,000
Stock option awards outstanding					329,498	275,922
Stock option awards, grant date fair value | $					$ 700,000	$ 300,000
Compensation cost recognized | $					$ 0	$ 0
Performance Contingent Stock Options Granted to Employees | Executive Officer
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option awards exercisable upon the achievement of performance goals					53,575
Weighted average exercise price | $ / shares					$ 3.42
Performance and Market Contingent Stock Options Granted to Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option awards outstanding					454,584	360,594
Stock option awards, grant date fair value | $					$ 1,000,000	$ 400,000
Compensation cost recognized | $					$ 0	$ 0
Performance and Market Contingent Stock Options Granted to Employees | Vesting 2
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option awards exercisable upon the achievement of performance goals					107,152
Vesting rights					(i) the closing of a financing where the Company sells shares of its equity securities to institutional investors at a minimum pre-money valuation, (ii) a change in control with minimum aggregate proceeds payable for the Company’s common stock at a minimum price per share, or (iii) either an initial public offering or an achievement of a minimum market capitalization, as measured by a trailing 30 day volume-weighted average price.
Performance and Market Contingent Stock Options Granted to Employees | Vesting 1
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option awards exercisable upon the achievement of performance goals					53,575
Vesting rights					(i) the closing of a financing where the Company sells shares of its equity securities to institutional investors at a minimum price per share, (ii) a change in control with aggregate proceeds payable for the Company’s common stock at a minimum price per share, or (iii) an initial public offering that becomes effective at a minimum specified price per share.
Trailing period for Initial public offering that becomes effective or achievement with a minimum market capitalization					30 days
Performance and Market Contingent Stock Options Granted to Employees | Executive Team
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted average exercise price | $ / shares					$ 3.42
Estimated fair value of stock options granted | $					$ 700,000
Performance and Market Contingent Stock Options Granted to Employees | Executive Team | Vesting 2
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option awards exercisable					160,727
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares vested					119,742
Restricted Stock | Executive Officer
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of restricted stock agreements | Agreement		2
Shares purchased		625,084
Shares vested		146,113
Restricted Stock | Executive Officer | Vesting 2
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares vested		359,229
Restricted Stock | Executive Officer | Vesting 1
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares vested		119,742
2018 Incentive Award Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance	4,289,936
Vesting period, options	4 years
Number of shares of common stock authorized for issuance					5,058,434
2018 Incentive Award Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expiration period from the date of grant, options	10 years
2018 Incentive Award Plan | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise price as a percentage of estimated fair value of the shares on the date of grant	100.00%
2013 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period, options			4 years
Exercise price | $ / shares				$ 3.42
Exercise price range, lower range limit | $ / shares				3.95
Exercise price range, upper range limit | $ / shares				$ 8.47
2013 Equity Incentive Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expiration period from the date of grant, options			10 years
2013 Equity Incentive Plan | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise price as a percentage of estimated fair value of the shares on the date of grant			100.00%
Exercise price as a percentage of estimated grant date fair value of shares for a 10% shareholder			110.00%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Shares Available for Grant
Shares Available for Grant, Beginning Balance	918,595
Shares Available for Grant, Retired in 2013 Plan	(378,875)
Shares Available for Grant, Authorized	4,289,936
Shares Available for Grant, Granted	(2,082,265)
Shares Available for Grant, Canceled	280,411
Shares Available for Grant, Ending Balance	3,027,802	918,595
Outstanding Options
Outstanding options, Begining Balance	4,196,213
Outstanding options, Granted	2,082,265
Outstanding options, Exercised	(501,329)
Outstanding options, Canceled	(276,618)
Outstanding options, Ending Balance	5,500,531	4,196,213
Outstanding options, Vested and exercisable at December 31, 2017	1,619,419
Outstanding options. Vested and expected to vest at December 31, 2017	5,500,531
Weighted-Average Exercise Price
Weighted-Average Exercise Price, Beginning Balance	$ 3.06
Weighted-Average Exercise Price, Options granted	13.20
Weighted-Average Exercise Price, Options exercised	3.17
Weighted-Average Exercise Price, Options canceled	5.82
Weighted-Average Exercise Price, Ending Balance	6.75	$ 3.06
Weighted-Average Exercise Price, Vested exercisable as of December 31, 2017	2.72
Weighted-Average Exercise Price, Options vested and expected to vest as of December 31, 2017	$ 6.75
Weighted-Average Remaining Contract Term
Balances at December 31, 2018	8 years 5 months 26 days
Exercisable, weighted average remaining contractual term (Year)	7 years 7 months 24 days
Options vested or expected to vest at end of period, weighted average remaining contractual term (Year)	8 years 5 months 26 days
Aggregate Intrinsic Value
Outstanding end of period, aggregate intrinsic value	$ 53,051
Exercisable, aggregate intrinsic value	21,929	$ 3,300
Options vested or expected to vest at end of period, aggregate intrinsic value	$ 53,051

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Valuation Assumption to Estimate Fair Value of Stock Options (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, Minimum	2.60%	1.80%	1.20%
Risk-free interest rate, Maximum	3.00%	2.20%	2.10%
Expected stock price volatility, Minimum	87.40%	77.00%	76.10%
Expected stock price volatility, Maximum	92.60%	82.00%	79.70%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term of options (in years)		5 years 7 months 6 days	5 years 3 months 18 days
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term of options (in years)	6 years 1 month 6 days	6 years 8 months 12 days	6 years 1 month 6 days

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Restricted Stock Activity (Details) - Restricted Stock	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares, Unvested at December 31, 2017 | shares	478,971
Shares, Vested | shares	(119,742)
Shares, Unvested at December 31, 2018 | shares	359,229
Weighted Average Grant Date Fair Value, Unvested at December 31, 2017	$ 4.57
Weighted Average Grant Date Fair Value, Granted	4.57
Weighted Average Grant Date Fair Value, Vested	4.57
Weighted Average Grant Date Fair Value, Unvested at December 30, 2018	$ 4.57

Stock-Based Compensation - Su_4

Stock-Based Compensation - Summary of Valuation Assumptions to Estimate Fair Values of Rights Granted (Details) - 2018 Employee Stock Purchase Plan	12 Months Ended
	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term of options (in years)	8 months 12 days
Risk-free interest rate	2.41%
Expected stock price volatility	73.18%

Stock-Based Compensation - Su_5

Stock-Based Compensation - Summary of Stock-based Compensation Expense (Details) - 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]
Stock-based compensation expense	$ 9,441	$ 3,034	$ 224
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	6,043	1,695	164
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 3,398	$ 1,339	$ 60

Warrants - Additional Informati

Warrants - Additional Information (Details) - $ / shares	Oct. 31, 2013	Dec. 31, 2018	Jan. 31, 2015	Jun. 30, 2013
Non Employee | Common Stock
Class Of Warrant Or Right [Line Items]
Warrants granted	96,610
Warrants granted, exercise price	$ 0.18
Unvested warrants expiration date	May 3, 2018
Warrants vested	9,425
Series A-1 Convertible Preferred Stock | Chief Executive Officer
Class Of Warrant Or Right [Line Items]
Warrants granted				192,823
Warrants granted, exercise price				$ 0.65
Series A-2 Convertible Preferred Stock | Chief Executive Officer
Class Of Warrant Or Right [Line Items]
Warrants granted			380,452	190,226
Warrants granted, exercise price			$ 0.66	$ 0.66
Convertible Preferred Stock | Chief Executive Officer
Class Of Warrant Or Right [Line Items]
Unvested warrants expiration date		Dec. 31, 2018

Net Loss per Common Share - Sch

Net Loss per Common Share - Schedule of Reconciliation of Numerators and Denominators Used In Computing Net Loss From Continuing Operations 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	$ (21,917)	$ (18,346)	$ (20,002)	$ (16,133)	$ (12,544)	$ (11,750)	$ (11,428)	$ (8,934)	$ (76,398)	$ (44,656)	$ (30,404)
Denominator:
Weighted average number of shares used in computing net loss per share, basic and diluted									28,269,907	3,197,516	2,662,841
Net loss per share, basic and diluted	$ (0.53)	$ (0.45)	$ (0.76)	$ (4.69)	$ (3.67)	$ (3.63)	$ (3.62)	$ (2.90)	$ (2.70)	$ (13.97)	$ (11.42)

Net Loss per Common Share - Sum

Net Loss per Common Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted 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]
Anti-dilutive securities excluded from computation of diluted net loss per share	6,591,409	34,842,052	27,276,579
Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share		28,159,724	24,620,615
Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	5,500,531	4,365,694	508,418
Early Exercised Common Stock Subject to Future Vesting
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	704,028	831,439	1,287,435
Restricted Stock Accounted For as Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	359,228	625,084
Warrants to Purchase Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share		763,501	763,501
Warrants to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share		96,610	96,610
Shares Subject to the 2018 ESPP
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	27,622

Net Loss per Common Share - Add

Net Loss per Common Share - Additional Information (Details)	12 Months Ended
	Dec. 31, 2018shares
Maximum
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Shares contingently issued	606,218

Defined Contribution Plan - Add

Defined Contribution Plan - Additional Information (Details) - 401(k) Plan	1 Months Ended
	Jan. 31, 2019
Defined Contribution Plan Disclosure [Line Items]
Percent of employer matching contribution of employees' salary	4.00%
Defined Contribution Plan, Sponsor Location [Extensible List]	country:US

Income Taxes - Schedule of Effe

Income Taxes - Schedule of Effective Tax Rate of Provision for Income Taxes Differs From Federal Statutory Rate (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Taxes at the U.S. statutory income tax rate	21.00%	34.00%	34.00%
State tax, net of federal benefit	0.90%
Other	(0.10%)	(2.20%)
Stock-based compensation	0.30%
General business credits	1.00%
Change in valuation allowance	(23.10%)	(13.30%)	(21.00%)
Non-deductible interest expense			(13.00%)
Change in income tax rate due to Tax Act		(18.50%)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Federal corporate income tax rate	21.00%	34.00%	34.00%
Percentage of net operating loss carry forward deductible from current year taxable income	80.00%
Decrease in deferred tax valuation allowance	$ 8,300,000
Net increase in the valuation allowance	17,600,000	$ 9,400,000
Unrecognized tax benefits that would impact effective tax rate	0	0
Liability recorded for potential interest or penalties	$ 0	$ 0
Internal Revenue Service
Income Taxes [Line Items]
Period of future cumulative changes in ownership interest of significant stockholders subject to annual limitation	3 years
Minimum percentage of ownership change as determined under regulations	50.00%
Additional net operating losses and research development credits subject to annual limitation	$ 1,600,000
Maximum
Income Taxes [Line Items]
Federal corporate income tax rate		35.00%

Income Taxes - Schedule of Comp

Income Taxes - Schedule of Components of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss	$ 29,926	$ 16,530
Research and development credits	3,865	1,879
Stock-based compensation	1,839	671
Contingent consideration	954
Accruals and other	1,040	895
Charitable contributions	253	330
Total deferred tax assets	37,877	20,305
Valuation allowance	$ (37,877)	(20,236)
Net deferred tax assets		69
Deferred tax liability		$ (69)

Income Taxes - Summary of Net O

Income Taxes - Summary of Net Operating Losses and Tax Credit Carryforwards (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Federal
Income Taxes [Line Items]
Net operating losses	$ 64,593	$ 64,136
Tax credits	$ 2,988
Operating losses carryforwards expiration years	indefinite	2030 - 2038
Tax credits carryforwards expiration years	2031 - 2039
State
Income Taxes [Line Items]
Net operating losses	$ 64,663
Tax credits	$ 2,692
Operating losses carryforwards expiration years	2030 - 2039
Tax credits carryforwards expiration years	Indefinite

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Gross unrecognized tax benefits at January 1	$ 3,065	$ 2,800
Additions for tax positions taken in the current year	753	478
Reductions for tax positions taken in the prior year	(104)	(213)
Gross unrecognized tax benefits at December 31	$ 3,714	$ 3,065

Subsequent Events - Additional

Subsequent Events - Additional information (Details)) $ in Millions	Feb. 28, 2019USD ($)ft²	Dec. 31, 2018
Subsequent Event [Line Items]
Operating lease, renewal term		4 years
Subsequent Event
Subsequent Event [Line Items]
Lease commencement date	Oct. 1, 2019
Lease, term	10 years
Operating lease, renewal term	8 years
Letter of credit, lease agreement | $	$ 0.9
Letter of credit, lease agreement description	term of the lease
Subsequent Event | CALIFORNIA
Subsequent Event [Line Items]
Lease agreement for office, research and development and laboratory space | ft²	62,655

Selected Quarterly Financial _3

Selected Quarterly Financial Data (Unaudited) - Schedule of Quarterly Financial Information (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]
Loss from operations	$ (22,808)	$ (19,377)	$ (20,798)	$ (16,482)	$ (12,787)	$ (12,083)	$ (11,698)	$ (9,040)	$ (79,465)	$ (45,608)	$ (18,844)
Net loss	$ (21,917)	$ (18,346)	$ (20,002)	$ (16,133)	$ (12,544)	$ (11,750)	$ (11,428)	$ (8,934)	$ (76,398)	$ (44,656)	$ (30,404)
Net loss per share, basic and diluted	$ (0.53)	$ (0.45)	$ (0.76)	$ (4.69)	$ (3.67)	$ (3.63)	$ (3.62)	$ (2.90)	$ (2.70)	$ (13.97)	$ (11.42)