IDEAYA Biosciences (IDYA) 10-K 24 Mar 20 2019 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Mar. 13, 2020	Jun. 28, 2019
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Trading Symbol	IDYA
Entity Registrant Name	IDEAYA Biosciences, Inc.
Entity Central Index Key	0001676725
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Shell Company	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		20,344,464
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Security Exchange Name	NASDAQ
Entity File Number	001-38915
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	47-4268251
Entity Address, Address Line One	7000 Shoreline Court
Entity Address, Address Line Two	Suite 350
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	650
Local Phone Number	443-6209
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Entity Public Float			$ 135.2
Document Annual Report	true
Document Transition Report	false
Documents Incorporated by Reference	Portions of the registrant’s definitive Proxy Statement relating to the 2019 Annual Meeting of Stockholders are incorporated herein by reference in Part III of this Annual Report on Form 10-K to the extent stated herein. The proxy statement will be filed with the Securities and Exchange Commission within 120 days of the registrant’s fiscal year ended December 31, 2019.

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 34,067	$ 20,505
Short-term marketable securities	64,889	69,456
Prepaid expenses and other current assets	2,698	706
Total current assets	101,654	90,667
Restricted cash	106	106
Long-term marketable securities	1,526
Property and equipment, net	4,642	5,152
Deferred offering costs		570
Right-of-use assets	5,057
Other non-current assets	16	46
Total assets	113,001	96,541
Current liabilities
Accounts payable	709	1,417
Accrued liabilities	5,023	3,583
Deferred rent		299
Lease liabilities	1,145
Other current liabilities	63	114
Total current liabilities	6,940	5,413
Deferred rent		1,572
Long-term lease liabilities	5,627
Other non-current liabilities	34	113
Total liabilities	12,601	7,098
Commitments and contingencies (Note 5)
Stockholders’ equity (deficit)
Preferred stock, $0.0001 par value, 10,000,000 shares authorized as of December 31, 2019 and no shares authorized as of December 31, 2018; no shares issued and outstanding as of December 31, 2019 and December 31, 2018
Common stock, $0.0001 par value, 300,000,000 shares authorized as of December 31, 2019 and 170,800,000 shares authorized as of December 31, 2018; 20,339,461 and 1,335,690 shares issued and outstanding as of December 31, 2019 and December 31, 2018	2
Additional paid-in capital	192,824	1,599
Accumulated other comprehensive income (loss)	65	(31)
Accumulated deficit	(92,491)	(50,516)
Total stockholders’ equity (deficit)	100,400	(48,948)
Total liabilities, redeemable convertible preferred stock and stockholders’ equity (deficit)	$ 113,001	96,541
Series A Redeemable Convertible Preferred Stock
Current liabilities
Temporary equity, value		40,735
Series B Redeemable Convertible Preferred Stock
Current liabilities
Temporary equity, value		$ 97,656

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	300,000,000	170,800,000
Common stock, shares issued	20,339,461	1,335,690
Common stock, shares outstanding	20,339,461	1,335,690
Series A Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock, par value	$ 0.0001	$ 0.0001
Redeemable convertible preferred stock, shares authorized	0	59,433,105
Redeemable convertible preferred stock, shares issued	0	5,794,727
Redeemable convertible preferred stock, shares outstanding	0	5,794,727
Redeemable convertible preferred stock, liquidation value	$ 0	$ 47,178
Series B Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock, par value	$ 0.0001	$ 0.0001
Redeemable convertible preferred stock, shares authorized	0	75,500,000
Redeemable convertible preferred stock, shares issued	0	7,345,067
Redeemable convertible preferred stock, shares outstanding	0	7,345,067
Redeemable convertible preferred stock, liquidation value	$ 0	$ 97,520

Statements of Operations and Co

Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating expenses
Research and development	$ 34,319	$ 31,749	$ 12,384
General and administrative	9,952	4,668	2,054
Total operating expenses	44,271	36,417	14,438
Loss from operations	(44,271)	(36,417)	(14,438)
Interest income	2,288	1,994	150
Other income (expense), net	8	77	2,426
Net loss	(41,975)	(34,346)	(11,862)
Unrealized gains (losses) on marketable securities	96	(30)	(1)
Comprehensive income (loss)	$ (41,879)	$ (34,376)	$ (11,863)
Net loss per share attributable to common stockholders, basic and diluted	$ (3.36)	$ (35.92)	$ (17.06)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted	12,496,957	956,252	695,372

Statements of Redeemable Conver

Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Redeemable Convertible Preferred Stock	Series A Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred StockLicense Agreement	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Redeemable convertible preferred stock, beginning balance at Dec. 31, 2016		$ 8,685
Redeemable convertible preferred stock, beginning balance, shares at Dec. 31, 2016		2,257,697
Issuance of redeemable convertible preferred stock, value			$ 17,399
Issuance of redeemable convertible preferred stock, shares			2,122,219
Redeemable convertible preferred stock, ending balance at Dec. 31, 2017		$ 26,084
Redeemable convertible preferred stock, ending balance, shares at Dec. 31, 2017		4,379,916
Beginning balance at Dec. 31, 2016	$ (4,132)					$ 0	$ 176	$ 0	$ (4,308)
Beginning balance, shares at Dec. 31, 2016						1,004,564
Issuance of common stock upon exercise of stock options	0					$ 0	0	0	0
Issuance of common stock upon exercise of stock options, shares						1,798
Early exercised common stock options	0					$ 0	0	0	0
Early exercised common stock options, shares						257,054
Vesting of early exercised common stock options and restricted stock	120					$ 0	120	0	0
Stock-based compensation	136					0	136	0	0
Other comprehensive income (loss)	(1)					0	0	(1)	0
Net loss	(11,862)					0	0	0	(11,862)
Ending balance at Dec. 31, 2017	(15,739)					$ 0	432	(1)	(16,170)
Ending balance, shares at Dec. 31, 2017						1,263,416
Issuance of redeemable convertible preferred stock, value			$ 14,651	$ 93,844	$ 3,812
Issuance of redeemable convertible preferred stock, shares			1,414,811	7,081,452	263,615
Redeemable convertible preferred stock, ending balance at Dec. 31, 2018		$ 138,391	$ 40,735	$ 97,656
Redeemable convertible preferred stock, ending balance, shares at Dec. 31, 2018		13,139,794	5,794,727	7,345,067
Issuance of common stock upon exercise of stock options	28					$ 0	28	0	0
Issuance of common stock upon exercise of stock options, shares						19,591
Early exercised common stock options	0					$ 0	0	0	0
Early exercised common stock options, shares						70,583
Repurchase of early exercised shares	$ 0					$ 0	0	0	0
Repurchase of early exercised shares, shares	(17,900)					(17,900)
Vesting of early exercised common stock options and restricted stock	$ 189					$ 0	189	0	0
Stock-based compensation	950					0	950	0	0
Other comprehensive income (loss)	(30)					0	0	(30)	0
Net loss	(34,346)					0	0	0	(34,346)
Ending balance at Dec. 31, 2018	(48,948)					$ 0	1,599	(31)	(50,516)
Ending balance, shares at Dec. 31, 2018						1,335,690
Conversion of redeemable convertible preferred stock into common stock		$ (138,391)
Conversion of redeemable convertible preferred stock into common stock, shares		(13,139,794)
Redeemable convertible preferred stock, ending balance, shares at Dec. 31, 2019		0	0	0
Conversion of redeemable convertible preferred stock into common stock	138,391					$ 1	138,390
Conversion of redeemable convertible preferred stock into common stock, shares						13,139,794
Issuance of common stock upon initial public offering, net of issuance cost	50,247					$ 1	50,246
Issuance of common stock upon initial public offering, net of issuance cost, shares						5,750,000
Issuance of common stock upon exercise of stock options	307						307
Issuance of common stock upon exercise of stock options, shares						135,563
Early exercised common stock options	0
Early exercised common stock options, shares						2,112
Repurchase of early exercised shares	$ 0
Repurchase of early exercised shares, shares	(23,698)					(23,698)
Vesting of early exercised common stock options and restricted stock	$ 114						114
Stock-based compensation	2,168						2,168
Other comprehensive income (loss)	96							96
Net loss	(41,975)								(41,975)
Ending balance at Dec. 31, 2019	$ 100,400					$ 2	$ 192,824	$ 65	$ (92,491)
Ending balance, shares at Dec. 31, 2019						20,339,461

Statements of Redeemable Conv_2

Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Series A Redeemable Convertible Preferred Stock
Stock issuance cost	$ 5	$ 14
Adjustment for the redeemable convertible preferred stock liability	3,137	$ 135
Series B Redeemable Convertible Preferred Stock
Stock issuance cost	$ 176

Statements of Cash Flows

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net loss	$ (41,975)	$ (34,346)	$ (11,862)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	1,245	886	391
Net amortization (accretion) of premiums (discounts) on marketable securities	(473)	(808)	(27)
Stock-based compensation	2,168	950	136
Issuance of Series B redeemable convertible preferred stock pursuant to license agreement		3,812
Change in fair value of redeemable convertible preferred stock liability		(70)	(2,426)
Landlord contributions for leasehold improvements		367
Loss (gain) on sale of property and equipment	14	(54)
Gain on sale of marketable securities	(8)	(11)
Changes in assets and liabilities
Prepaid expenses and other assets	(1,963)	(243)	331
Right-of-use assets	1,093
Accounts payable	(195)	322	254
Accrued and other liabilities	2,030	1,672	1,015
Deferred rent		(97)	(36)
Lease liabilities	(1,249)
Net cash used in operating activities	(39,313)	(27,620)	(12,224)
Cash flows from investing activities
Purchases of property and equipment, net	(1,353)	(1,708)	(1,757)
Purchases of marketable securities	(88,004)	(133,301)	(13,470)
Maturities of marketable securities	73,528	65,830	6,300
Sales of marketable securities	18,094	6,000
Net cash provided by (used in) investing activities	2,265	(63,179)	(8,927)
Cash flows from financing activities
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs		105,358	17,264
Proceeds from issuance of common stock in initial public offering, net of issuance costs	50,321
Proceeds from exercise of common stock options	289	91	226
Payments of deferred offering costs		(71)
Net cash provided by financing activities	50,610	105,378	17,490
Net increase (decrease) in cash, cash equivalents and restricted cash	13,562	14,579	(3,661)
Cash, cash equivalents and restricted cash, at beginning of period	20,611	6,032	9,693
Cash, cash equivalents and restricted cash, at end of period	34,173	20,611	6,032
Reconciliation of cash, cash equivalents and restricted cash
Cash and cash equivalents	34,067	20,505	5,961
Restricted cash	106	106	71
Cash, cash equivalents and restricted cash, at end of period	34,173	20,611	6,032
Supplemental disclosure of cash flow information:
Cash paid for income taxes	1	1
Cash paid for interest	91	99	19
Supplemental non-cash investing and financing activities:
Leasehold improvements acquired through tenant improvement allowance			1,637
Unpaid deferred offering costs		499
Vesting of early exercised options and restricted stock	114	189	120
Purchases of property and equipment in accounts payable and accrued liabilities		604	70
Extinguishment of redeemable convertible preferred stock liability		3,137	$ 135
Conversion of redeemable convertible preferred stock into common stock	$ 138,391
Series B Redeemable Convertible Preferred Stock
Supplemental non-cash investing and financing activities:
Issuance of Series B redeemable convertible preferred stock pursuant to license agreement		$ 3,812

Organization

Organization	12 Months Ended
	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	1. Organization Description of the Business IDEAYA Biosciences, Inc. (the “Company”) is an oncology-focused precision medicine company committed to the discovery and development of targeted therapeutics for patient populations selected using molecular diagnostics. The Company is headquartered in South San Francisco, California and was incorporated in the State of Delaware in June 2015. To date, the Company has been primarily engaged in business planning, research, development, recruiting and raising capital. Initial Public Offering On May 22, 2019, the Company’s registration statement on Form S-1 (File No. 333-231081) relating to its initial public offering (“IPO”) of common stock became effective. The IPO closed on May 28, 2019 at which time the Company issued 5,750,000 shares of its common stock at a price of $10.00 per share, which included shares issued upon the underwriters’ exercise of their overallotment option to purchase 750,000 additional shares. In addition, upon closing the IPO, all outstanding shares of the redeemable convertible preferred stock converted into 13,139,794 shares of common stock. The Company received an aggregate of $50.2 million in cash, net of underwriting discounts and commissions of $4.0 million, and after deducting offering costs of $3.3 million. Liquidity The Company has incurred significant losses and negative cash flows from operations in all periods since inception and had an accumulated deficit of $92.5 million as of December 31, 2019. The Company has historically financed its operations primarily through the sale of redeemable convertible preferred stock, common stock and convertible notes. To date, none of the Company’s product candidates have been approved for sale, and the Company has not generated any revenue since inception. Management expects operating losses to continue and increase for the foreseeable future, as the Company progresses into clinical development activities for its lead product candidates. The Company’s prospects are subject to risks, expenses and uncertainties frequently encountered by companies in the biotechnology industry as discussed under Risks and Uncertainties in Note 2. While the Company has been able to raise multiple rounds of financing, there can be no assurance that in the event the Company requires additional financing, such financing will be available on terms which are favorable or at all. Failure to generate sufficient cash flows from operations, raise additional capital or reduce certain discretionary spending would have a material adverse effect on the Company’s ability to achieve its intended business objectives. As of December 31, 2019, the Company had cash, cash equivalents and marketable securities of $100.5 million. Management believes that the Company’s current cash, cash equivalents and marketable securities will be sufficient to fund its planned operations for at least 12 months from the date of the issuance of these financial statements.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The financial statements and accompanying notes have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”). Reverse Stock Split In May 2019, the Company’s board of directors approved a 1-for-10.2564 reverse stock split of the Company’s common stock and redeemable convertible preferred stock, which was effected on May 21, 2019. The par value and authorized shares of the common stock and redeemable convertible preferred stock were not adjusted as a result of the reverse stock split. All issued and outstanding common stock, options to purchase common stock and per share amounts contained in these financial statements have been retroactively adjusted to give effect to the reverse stock split for all periods presented. Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of expenses during the reporting period. Such estimates include useful lives of property and equipment, determination of the discount rate for operating leases, accruals for research and development activities, stock-based compensation, income taxes, and prior to the IPO, the valuation of the redeemable convertible preferred stock liability. Actual results could differ from those estimates. Segments The Company operates and manages its business as one operating and reportable segment, which is the business of research and development for oncology-focused precision medicine. The Company’s chief executive officer, who is the chief operating decision maker, reviews financial information on an aggregate basis for purposes of allocating resources and evaluating financial performance. All of the Company's long-lived assets are located in the United States. Risks and Uncertainties The Company operates in a dynamic and highly competitive industry and is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, protection of proprietary technology, dependence on key personnel, contract manufacturer and contract research organizations, compliance with government regulations and the need to obtain additional financing to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical studies and clinical trials and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance and reporting. The Company believes that changes in any of the following areas could have a material adverse effect on the Company’s future financial position, results of operations, or cash flows; ability to obtain future financing; advances and trends in new technologies and industry standards; results of clinical trials; regulatory approval and market acceptance of the Company’s products; development of sales channels; certain strategic relationships; litigation or claims against the Company base on intellectual property, patent, product, regulatory, or other factors; and the Company’s ability to attract and retain employees necessary to support its growth. Products developed by the Company require approvals from the U.S. Food and Drug Administration (“FDA”) or other international regulatory agencies prior to commercial sales. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained or maintained, that the products will receive the necessary approvals, or that any approved products will be commercially viable. If the Company was denied approval, approval was delayed or the Company was unable to maintain approval, it could have a materially adverse impact on the Company. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from other pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees, consultants and other third parties. In December 2019, a novel strain of coronavirus was reported in Wuhan, China. The extent of the impact of the coronavirus outbreak on the Company’s business will depend on certain developments, including the duration and spread of the outbreak and the extent and severity of the impact on the Company’s clinical trial activities and suppliers, all of which are uncertain and cannot be predicted. At this point, the extent to which the coronavirus outbreak may materially impact the Company’s financial condition, liquidity or results of operations is uncertain. The Company has expended and will continue to expend substantial funds to complete the research, development and clinical testing of product candidates. The Company also will be required to expend additional funds to establish commercial-scale manufacturing arrangements and to provide for the marketing and distribution of products that receive regulatory approval. The Company may require additional funds to commercialize its products. The Company is unable to entirely fund these efforts with its current financial resources. If adequate funds are unavailable on a timely basis from operations or additional sources of financing, the Company may have to delay, reduce the scope of or eliminate one or more of its research or development programs which would materially and adversely affect its business, financial condition and operations. Concentration of Credit Risk Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash, cash equivalents and marketable securities. Substantially all the Company’s cash is held by one financial institution that management believes is of high credit quality. Such deposits may, at times, exceed federally insured limits. The Company’s investment policy addresses credit ratings, diversification, and maturity dates. The Company invests its cash equivalents and marketable securities in money market funds, U.S. government securities, commercial paper, corporate bonds, and asset-backed securities. The Company limits its credit risk associated with cash equivalents and marketable securities by placing them with banks and institutions it believes are highly creditworthy and in highly rated investments and, by policy, limits the amount of credit exposure with any one commercial issuer. The Company has not experienced any credit losses on its deposits of cash, cash equivalents or marketable securities. Deferred Initial Public Offering Costs Deferred offering costs, consisting of legal, accounting and other fees and costs relating to the Company’s IPO, are capitalized and recorded on the balance sheet. As of December 31, 2018, $0.6 million of deferred offering costs were recorded on the balance sheet. During the year ended December 31, 2019, an additional $2.7 million in deferred offering costs were incurred. In connection with the IPO, all deferred offering costs reduced the gross proceeds from the IPO and were recorded within additional paid-in capital on the accompanying balance sheet as of December 31, 2019. Cash and Cash Equivalents Cash equivalents that are readily convertible to cash are stated at cost, which approximates fair value. The Company considers all highly liquid investments purchased with an original or remaining maturity of three months or less at the date of purchase to be cash equivalents. Restricted Cash Restricted cash as of December 31, 2019 and December 31, 2018 consisted of cash balances held as security in connection with the Company’s facility lease agreement in South San Francisco, California. The balances are classified as long-term assets on the Company’s balance sheet. Marketable Securities Marketable securities are investments in marketable securities with maturities greater than three months at the time of purchase. The Company determines the appropriate classification of its investments in marketable securities at the time of purchase and reevaluates such designation at each balance sheet date. The Company has classified and accounted for its marketable securities as available-for-sale. After consideration of the Company’s risk versus reward objectives and liquidity requirements, the Company may sell these securities prior to their stated maturities. The Company classifies highly liquid securities with maturities beyond 12 months as long-term marketable securities in the balance sheet. These securities are carried at fair value as determined based upon quoted market prices or pricing models for similar securities. Unrealized gains and losses, if any, are excluded from earnings and are reported as a component of accumulated other comprehensive income (loss). 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, if any, on available-for-sale securities are included in other income (expense), net. The cost of securities sold is based on the specific identification method. Interest and dividends on securities classified as available-for-sale are included in interest income. The Company did not identify any of its marketable securities as other-than-temporarily impaired as of December 31, 2019 . Fair Value of Financial Instruments The carrying amounts of the Company’s certain financial instruments, including cash equivalents, accounts payable and accrued liabilities approximate fair value due to their relatively short maturities and market interest rates if applicable. The carrying amount of the redeemable convertible preferred stock liability represents its fair value. Refer to Note 3 for details on the fair value of marketable securities. Property and Equipment, Net Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation is computed using the straight-line method over the estimated useful lives of the assets, which is generally between three and five years. Leasehold improvements are stated at cost and amortized over the shorter of the useful lives of the assets or the lease term Impairment of Long-Lived Assets The Company reviews property and equipment for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset or asset group may not be recoverable. Recoverability is measured by comparison of the carrying amount of the asset or asset group to the future net cash flows which the asset or asset group is expected to generate. If such asset or asset group is considered to be impaired, the impairment to be recognized is measured by the amount by which the carrying amount of the asset or asset group exceeds the fair value of the asset or asset group. There have been no such impairments of long-lived assets for the years ended December 31, 2019 and December 31, 2018. Leases The Company leases its facilities and meets the requirements to account for these leases as operating leases. For the year ended December 31, 2018, for facility leases that contain rent escalations or rent concession provisions, the Company records its lease expense during the lease term on a straight-line basis over the term of the lease. As of December 31, 2018, the Company recorded the difference between the rent paid and the straight-line rent as a deferred rent liability. As of December 31, 2018, leasehold improvements funded by landlord incentives or allowances were recorded as leasehold improvement assets and a corresponding deferred rent liability. The leasehold improvement asset is amortized over the lesser of the term of the lease or life of the asset. For the year ended December 31, 2018, the deferred rent liability is amortized on a straight-line basis as a reduction to rent expense over the term of the lease agreement. Upon adoption of ASC 842, Leases . ROU assets and lease liabilities are recognized based on the present value of the future lease payments over the lease term at commencement date. As most of the Company’s leases do not provide an implicit rate, the Company uses its incremental borrowing rate based on the information available at commencement date in determining the present value of future payments. The ROU asset also includes any lease payments made to the lessor at or before the commencement date, minus lease incentives received, and initial direct costs incurred. The Company’s lease terms may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Lease expense for lease payments is recognized on a straight-line basis over the lease term. For lease agreements entered into or reassessed after the adoption of ASC 842, the Company combines lease and nonlease components. Redeemable Convertible Preferred Stock The Company recorded all shares of redeemable convertible preferred stock at their respective fair values on the dates of issuance, net of issuance costs. The redeemable convertible preferred stock was recorded outside of permanent equity because while it was not mandatorily redeemable, in certain events considered not solely within the Company’s control, such as a merger, acquisition, or sale of all or substantially all of the Company’s assets (each, a “deemed liquidation event”), the convertible preferred stock would become redeemable at the option of the holders of at least a majority of the then outstanding such shares. The Company did not adjust the carrying values of the redeemable convertible preferred stock to its liquidation preference because a deemed liquidation event obligating the Company to pay the liquidation preferences to holders of shares of redeemable convertible preferred stock was not probable of occurring. Subsequent adjustments to the carrying values to the liquidation preferences would have been made only when it became probable that such a deemed liquidation event would occur. Redeemable Convertible Preferred Stock Liability The obligation to issue additional shares of Series A redeemable convertible preferred stock at future dates pursuant to a preferred stock purchase agreement was determined to be a freestanding instrument that should be accounted for as a liability. At initial recognition, the Company recorded the redeemable convertible preferred stock liability on the balance sheet at its fair value. The liability was until it was extinguished upon issuance of Series A redeemable convertible preferred stock in January 2018. Research and Development Expenses Research and development expenses consist of compensation costs, employee benefit costs, costs for contract manufacturing organizations (“CMOs”), costs for contract research organizations (“CROs”), costs for sponsored research, consulting costs, costs for laboratory supplies, costs for product licenses, facility-related expenses and depreciation. All research and development costs are charged to research and development expenses within the statements of operations and comprehensive loss as incurred. Payments associated with licensing agreements to acquire exclusive licenses to develop, use, manufacture and commercialize products that have not reached technological feasibility and do not have alternate commercial use are also expensed as incurred. Payments made to third parties under these arrangements in advance of the performance of the related services by the third parties are recorded as prepaid expenses until the services are rendered. Accrued Research and Development The Company has entered into various agreements with CMOs and CROs. The Company’s research and development accruals are estimated based on the level of services performed, progress of the studies, including the phase or completion of events, and contracted costs. The estimated costs of research and development provided, but not yet invoiced, are included in accrued liabilities on the balance sheet. If the actual timing of the performance of services or the level of effort varies from the original estimates, the Company will adjust the accrual accordingly. Payments made to CMOs and CROs under these arrangements in advance of the performance of the related services are recorded as prepaid expenses and other current assets until the services are rendered. Stock-Based Compensation The Company accounts for stock-based compensation arrangements with employees in accordance with ASC 718, Stock Compensation. For the years ended December 31, 2018 and December 31, 2017, the Company accounted for stock options issued to non-employees based on the estimated fair value of the awards using the Black-Scholes option pricing model in accordance with ASC 505-50, Equity-Based Payment to Non-employees Upon adoption of ASU 2018-07, Compensation–Stock Compensation (Topic 718): Improvements to Non-employee Share–Based Payment Accounting ASC 718, Stock Compensation. Income Taxes The Company accounts for income taxes using the asset and liability method whereby deferred tax assets and liabilities are determined based on differences between financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that are currently in effect unless such rate is expected to be different when the deferred item reverses. Valuation allowances are established where necessary to reduce deferred tax assets to the amounts expected to be realized. Deferred tax assets and liabilities are classified as noncurrent on the balance sheet. The Company recognizes the effect of income tax positions only if those positions are more likely than not of being sustained. Recognized income tax positions are measured at the largest amount that has a greater than 50% likelihood of being realized. Changes in recognition or measurement are reflected in the period in which the change in judgment occurs. The Company records interest and penalties related to unrecognized tax benefits in interest expense and other expense, respectively. Comprehensive Loss Comprehensive loss represents all changes in stockholders’ equity (deficit) except those resulting from and distributions to stockholders. Net Loss per Share Attributable to Common Stockholders Basic net loss per common share is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of common stock outstanding during the period, without consideration of potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common stock and potentially dilutive securities outstanding for the period. For purposes of the diluted net loss per share calculation, redeemable convertible preferred stock, stock options and restricted stock that is subject to repurchase at the original purchase price are considered to be potentially dilutive securities. Basic and diluted net loss attributable to common stockholders per share is presented in conformity with the two-class method required for participating securities as the redeemable convertible preferred stock is considered a participating security because it participates in dividends with common stock. The Company also considers the shares issued upon the early exercise of stock options subject to repurchase to be participating securities, because holders of such shares have non-forfeitable dividend rights in the event a dividend is paid on common stock. The holders of all series of redeemable convertible preferred stock and the holders of early exercised shares subject to repurchase do not have a contractual obligation to share in the Company’s losses. As such, the net loss was attributed entirely to common stockholders. Because the Company has reported a net loss for all periods presented, diluted net loss per common share is the same as basic net loss per common share for those periods. Recent Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (“FASB”) under its accounting standard codifications (“ASC”) or other standard setting bodies and adopted by the Company as of the specified effective date, unless otherwise discussed below. Recently Adopted Accounting Pronouncements In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) Leases Codification Improvements to Topic 842, Leases Leases (Topic 842): Targeted Improvements Leases (Topic 842): Codification Improvements. for its operating leases as of January 1, 2019, including deferred rent of $1.9 million The adoption of these ASUs did not have any impact on the statements of operations and comprehensive loss and statements of cash flows. See Note 5 for more information related to the Company’s lease obligations. In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260); Distinguishing Liabilities from Equity (Topic 480); Derivatives and Hedging (Topic 815): (Part I) Accounting for Certain Financial Instruments with Down Round Features, (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception In February 2018, the FASB issued ASU 2018-02, Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Non-employee Share-Based Payment Accounting New Accounting Pronouncements Not Yet Adopted In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which requires the measurement and recognition of expected credit losses for financial assets held at amortized cost. ASU 2016-13 replaces the existing incurred loss impairment model with an expected loss model. It also eliminates the concept of other-than-temporary impairment and requires credit losses related to available-for-sale debt securities to be recorded through an allowance for credit losses rather than as a reduction in the amortized cost basis of the securities. These changes will result in earlier recognition of credit losses. For public business entities, this ASU is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. The FASB subsequently issued supplemental guidance to ASC 326 within ASU 2019-05, Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief , ASU 2019-10, Financial Instruments—Credit Losses (Topic 326), Derivatives and Hedging (Topic 815), and Leases (Topic 842) and ASU 2019-11, Codification Improvements to Topic 326, Financial Instruments—Credit Losses . ASU 2019-05 provides an option to irrevocably elect the fair value option for certain financial assets previously measured at amortized cost basis. ASU 2019-10 extended the effectiveness of Topic 326 for smaller reporting companies until fiscal years beginning after December 31, 2020. Early adoption is permitted. The Company is currently evaluating the impact the adoption of these ASUs will have on its financial statements and related disclosures, and the adoption timing of these ASUs In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement , which modifies the disclosure requirements on fair value measurements. ASU 2018-13 removes the requirement to disclose: the amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy; the policy for timing of transfers between levels; and the valuation processes for Level 3 fair value measurements. For public business entities, this ASU is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. The Company is currently evaluating the impact the adoption of this ASU will have on its financial statements and related disclosures. In August 2018, the FASB issued ASU 2018-15, Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. ASU 2018-15 requires that certain implementation costs incurred in a cloud computing arrangement be deferred and recognized over the term of the arrangement. For public business entities, this ASU is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. The Company is currently evaluating the impact the adoption of this ASU will have on its financial statements and related disclosures. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. ASU 2019-12 simplifies the accounting for income taxes by removing certain exceptions to the general principles in Topic 740 and improves consistent application of and simplify GAAP for other areas of Topic 740 by clarifying existing guidance. For public business entities, this ASU is effective for fiscal years beginning after December 15, 2020, and interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the impact the adoption of this ASU will have on its financial statements and related disclosures.

Fair Value Measurement

Fair Value Measurement	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurement	3. Fair Value Measurement The Company applies fair value accounting for all financial assets and liabilities and non-financial assets and liabilities that are recognized or disclosed at fair value in the financial statements on a recurring basis. Fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, a three-tier fair value hierarchy has been established, which prioritizes the inputs used in measuring fair value as follows: Level 1—Observable inputs, such as quoted prices in active markets for identical assets or liabilities at the measurement date . Level 2—Observable inputs other than Level 1 prices such as quoted prices for similar assets or liabilities, quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities . Level 3—Unobservable inputs which reflect management’s best estimate of what market participants would use in pricing the asset or liability at the measurement date. Consideration is given to the risk inherent in the valuation technique and the risk inherent in the inputs to the model . In determining fair value, the Company utilizes valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible as well as considers counterparty credit risk in its assessment of fair value. The Company’s Level 3 liabilities consist of the redeemable convertible preferred stock liability. The determination of the fair value of the redeemable convertible preferred stock liability is discussed in Note 8. As of December 31, 2019, financial assets and liabilities measured and recognized at fair value are as follows (in thousands): December 31, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Assets U.S. government securities Level 1 $ 24,973 $ 32 $ — $ 25,005 Corporate bonds Level 2 39,185 37 (4 ) 39,218 Commercial paper Level 2 2,192 — — 2,192 Marketable securities 66,350 69 (4 ) 66,415 Money market funds ( 1) Level 1 34,008 — — 34,008 Total fair value of assets $ 100,358 $ 69 $ (4 ) $ 100,423 (1) Included in cash and cash equivalents on the balance sheet As of December 31, 2018, financial assets measured and recognized at fair value are as follows (in thousands): December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Assets U.S. government securities Level 1 $ 9,983 $ — $ (1 ) $ 9,982 Corporate bonds Level 2 18,820 — (19 ) 18,801 Commercial paper Level 2 29,699 — — 29,699 Asset-backed securities Level 2 10,984 — (10 ) 10,974 Marketable securities 69,486 — (30 ) 69,456 Money market funds ( 1) Level 1 20,419 — — 20,419 Total fair value of assets $ 89,905 $ — $ (30 ) $ 89,875 (1) Included in cash and cash equivalents on the balance sheet As of December 31, 2019, marketable securities had a remaining maturity of twenty months or less. As of December 31, 2018, marketable securities had a remaining maturity of twelve months or less. There were no financial liabilities measured and recognized at fair value as of December 31, 2019 and December 31, 2018. The following tables set forth the changes in the fair value of Level 3 financial liabilities (in thousands): Redeemable Convertible Preferred Stock Liability Fair value as of January 1, 2018 $ 3,207 Issuance of redeemable convertible preferred stock (3,137 ) Change in fair value included in other income (expense), net (70 ) Fair value as of December 31, 2018 $ — The Company used the Black-Scholes option pricing model (“OPM”) to estimate the fair value of the redeemable convertible preferred stock liability (see Note 8).

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2019
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Components	4. Balance Sheet Components Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): Useful Life As of December 31, (In Years) 2019 2018 Laboratory equipment 5 $ 4,034 $ 3,335 Computer equipment 3 117 117 Software 3 118 109 Leasehold improvements Shorter of useful life or lease term 2,581 2,570 Furniture and fixtures 5 308 304 Total property and equipment 7,158 6,435 Less: Accumulated depreciation and amortization (2,516 ) (1,283 ) Property and equipment, net $ 4,642 $ 5,152 Depreciation and amortization expense was $1.2 million and $0.4 million for the years ended December 31, 2019, Accrued Liabilities Accrued liabilities consisted of the following (in thousands): As of December 31, 2019 2018 Accrued salaries and benefits $ 1,733 $ 1,279 Accrued research and development expenses 2,787 1,095 Legal and professional fees 457 999 Other 46 210 Accrued liabilities $ 5,023 $ 3,583

Leases, Lease Commitments and C

Leases, Lease Commitments and Contingencies	12 Months Ended
	Dec. 31, 2019
Leases [Abstract]
Leases, Lease Commitments and Contingencies	5. Leases, Lease Commitments and Contingencies Operating Leases The Company leases its laboratory and office facilities in South San Francisco, California, and through September 30, 2019 in San Diego, California under non-cancelable operating leases with expiration dates in July 2024 and March 2020, respectively. On September 30, 2019, the Company and the landlord of the laboratory and office facilities in South San Francisco entered into a second amendment (“Second Amendment”) to lease additional office spaces at the same location. The Company will account for the Second Amendment as a separate contract and will recognize a related ROU asset and lease liability on the lease commencement date of the additional office spaces, which is currently expected in early 2020. On September 30, 2019, the Company and the landlord of the laboratory and office facilities in San Diego agreed to terminate the lease. No penalty was incurred as a result of the termination. Accordingly, the Company derecognized the remaining balances of the ROU asset and lease liability. As part of the lease agreement for the laboratory and office facilities in South San Francisco, the Company was provided a tenant improvement allowance of $1.6 million. In May 2018, the Company entered into an amendment to the lease agreement to expand the size of the laboratory and office facilities leased in South San Francisco, California by adding additional space to the lease commencing June 1, 2018. As part of the amendment, the landlord provided an additional tenant improvement allowance of $0.4 million. The Company was amortizing the tenant improvement allowances over the term of the lease agreement through December 31, 2018. Rent expense was $1.4 million and $0.8 million for the years ended December 31, 2018 and December 31, 2017, respectively. As of December 31, 2018, $1.9 million of deferred rent representing future minimum rental payments for leases with scheduled rent escalations and tenant improvement allowances was included in current and long-term liabilities. Future minimum lease payments under the non-cancelable operating leases as of December 31, 2018 are as follows (in thousands): As of December 31, 2018: Operating Leases 2019 $ 1,711 2020 1,611 2021 1,604 2022 1,646 2023 1,690 Thereafter 1,441 Total lease payments $ 9,703 The maturities of operating lease liabilities as of December 31, 2019 are as follows (in thousands): As of December 31, 2019 Operating Leases 2020 $ 1,564 2021 1,604 2022 1,646 2023 1,690 2024 1,441 Total lease payments 7,945 Less: Interest (1,173 ) Present value of lease liabilities $ 6,772 Amounts recognized on the balance sheet Current lease liabilities $ 1,145 Long-term lease liabilities 5,627 Total lease liabilities $ 6,772 Operating lease cost was $1.5 million for the year ended December 31, 2019. As of December 31, 2019, the ROU assets of $5.1 million are included in non-current assets on the balance sheet, and lease liabilities of $6.8 million are included in current liabilities and non-current liabilities on the balance sheet. As of December 31, 2019, the remaining term for the operating lease in South San Francisco, California is 4.6 years, and the discount rate used to measure the lease liability for such operating lease upon recognition is 7.0%. The Company has one right to extend the lease term of the operating leases in South San Francisco for two years by giving the landlord written notice. The remaining term does not include additional two years, as the Company assessed at commencement date that it was not reasonably certain to extend the lease term. During the year ended December 31, 2019, cash paid for amounts included in operating lease liabilities of $1.7 million is included in cash flows from operating activities on the statement of cash flows. Contingencies From time to time, the Company may be involved in litigation related to claims that arise in the ordinary course of its business activities. The Company accrues for these matters when it is probable that future expenditures will be made and these expenditures can be reasonably estimated. As of December 31, 2019, the Company does not believe that any such matters, individually or in the aggregate, will have a material adverse effect on the Company’s financial position, results of operations or cash flows . Indemnification The Company enters into standard indemnification arrangements in the ordinary course of business with vendors and other parties. Pursuant to these arrangements, the Company indemnifies, holds harmless and agrees to reimburse the indemnified parties for losses suffered or incurred by the indemnified party. The term of these indemnification agreements is generally perpetual any time after the execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these arrangements is not determinable. The Company has never incurred costs to defend lawsuits or settle claims related to these indemnification agreements. As a result, the Company believes the fair value of these agreements is not material .

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	6. Income Taxes No provision for income taxes was recorded for the years ended December 31, 2019, December 31, 2018 and December 31, 2017. The Company has incurred net operating losses only in the United States since its inception. The Company has not reflected any benefit of such net operating loss carryforwards in the financial statements. The provision for income taxes differs from the amount expected by applying the federal statutory rate to the loss before taxes as follows: Year Ended December 31, 2019 2018 2017 Federal statutory income tax rate 21.0 % 21.0 % 34.0 % State income taxes 7.0 % 6.9 % 6.9 % Change in valuation allowance (31.4 %) (29.4 %) (29.2 %) Research tax credits 3.6 % 2.7 % 2.9 % Other permanent differences (0.4 %) (1.3 %) (2.1 %) Change in fair value of redeemable convertible preferred stock liability 0.2 % 0.1 % 7.0 % Remeasurement of deferred tax due to tax law change — % — % (19.5 %) Provision for income taxes 0.0 % 0.0 % 0.0 % On December 22, 2017, the U.S. government enacted comprehensive tax legislation through the Tax Act. The Tax Act significantly revises the future ongoing U.S. corporate income tax by, among other things, lowering the U.S. corporate income tax rates and implementing a territorial tax system. The corporate tax rate was reduced from 34% to 21% for tax years beginning after December 31, 2017. Under the Tax Act, net operating losses arising after December 31, 2017 do not expire and cannot be carried back. However, the Tax Act limits the amount of net operating losses that can be used annually to 80% of taxable income for net operating losses arising in periods beginning after December 31, 2017. Existing net operating losses arising in years ending on or before December 31, 2017 are not affected by these provisions. The tax effects of temporary differences and carryforwards of the deferred As of December 31, 2019 2018 Deferred tax assets: Net operating loss carryforwards $ 23,947 $ 12,633 Research and development credit carryforwards 2,995 1,458 Lease liability 1,895 — Intangible assets 1,809 1,913 Accruals and reserves 436 1,048 Stock-based compensation 407 — Gross deferred tax assets 31,489 17,052 Less: Valuation allowance (29,462 ) (16,289 ) Deferred tax assets, net of valuation allowance 2,027 763 Deferred tax liabilities: Right-of-use assets (1,415 ) — Property and equipment (612 ) (724 ) Stock-based compensation — (39 ) Net deferred tax assets $ — $ — The Company has established a full valuation allowance against its deferred tax assets due to the uncertainty surrounding the realization of such 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. As of December 31, 2019, the Company had net operating loss carryforwards of $85.7 million available to reduce future taxable income, if any, for federal income tax purposes. As of December 31, 2019, the Company had net operating loss carryforwards of $84.6 million available to reduce future taxable income, if any, for California state income tax purposes. If not utilized, the federal carryforwards of $18.9 million and state carryforwards of $84.6 million will begin to expire in 2036. The federal net operating loss carryforwards of $66.8 million arising after December 31, 2017 do not expire. The Company also had federal and state research and development credit carryforwards of $1.9 million and $2.2 million, respectively, as of December 31, 2019. The federal credits will expire starting in 2036 if not utilized and the state research credit can be carried forward indefinitely. The Tax Reform Act of 1986 limits the use of net operating loss carryforwards in certain situations where changes occur in the stock ownership of a company. The annual limitation may result in the expiration of net operating losses and credits before utilization. The Company does not expect any previous ownership changes, as defined under Sections 382 and 383 of the Internal Revenue Code of 1986, as amended, to result in a limitation that will materially reduce the total amount of net operating loss carryforwards and credits that can be utilized. The Company performed a Section 382 analysis through December 31, 2018. As a result, the Company reduced its deferred tax assets and related valuation allowance to take into account net operating losses and research and development credits that would expire unused. The Company does not expect any ownership changes, as defined under Sections 382 and 383 of the Internal Revenue Code of 1986, as amended, during the year ended December 31, 2019 to result in a limitation that would materially reduce the total amount of net operating loss carryforwards and credits that can be utilized. Related to unrecognized tax benefits noted below, the Company accrued no penalties or interest during the years ended December 31, 2019, December 31, 2018 and December 31, 2017. The Company does not expect its unrecognized tax benefit balance to change materially over the next 12 months. The Company had $0.7 million and $0.4 million of unrecognized tax benefits as of December 31, 2019 and December 31, 2018, respectively. The following table summarizes the activity related to the Company’s unrecognized tax benefits (in thousands). Balance as of January 1, 2018 $ 174 Increase related to prior year tax positions 12 Increase related to current year tax positions 213 Balance as of December 31, 2018 $ 399 Increase related to prior year tax positions 59 Increase related to current year tax positions 288 Balance as of December 31, 2019 $ 746 The Company files income tax returns in the U.S. federal jurisdiction and in California. For jurisdictions in which tax filings have been filed, all tax years remain open for examination by the federal and California state authorities for three and four years, respectively, from the date of utilization of any net operating losses or credits.

Redeemable Convertible Preferre

Redeemable Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2019
Temporary Equity Disclosure [Abstract]
Redeemable Convertible Preferred Stock	7. Redeemable Convertible Preferred Stock In March 2016, the Company issued 1,630,910 shares of Series A redeemable convertible preferred stock at $8.1415 per share for gross proceeds of $13.3 million, and the Company issued 626,787 In February 2017, the Company issued an additional 2,122,219 shares of Series A redeemable convertible preferred stock at $8.1415 per share for gross proceeds of $17.3 million. In January 2018, the Company issued 1,414,811 shares of Series A redeemable convertible preferred stock at $8.1415 per share for gross proceeds of $11.5 million. In January 2018, the Company issued 6,328,265 shares of a newly authorized series of preferred stock, Series B redeemable convertible preferred stock, at $13.2769 per share for gross proceeds of $84.0 million. Subsequently, in March 2018, the Company issued additional 753,187 shares of Series B redeemable convertible preferred stock at $13.2769 per share for gross proceeds of $10.0 million. In September 2018, the Company issued 263,615 shares of Series B redeemable convertible preferred stock with a total fair value of $3.8 million to an affiliate of Novartis in connection with the license agreement with Novartis (see note 13). As of December 31, 2019 and December 31, 2018, the Company’s certificate of incorporation authorized the Company to issue zero and 134,933,105 shares of redeemable convertible preferred stock, respectively, at a par value of $0.0001 per share. Issued and outstanding redeemable convertible preferred stock and its principal terms as of December 31, 2018 were as follows (in thousands, except share and per share amounts) Redeemable Convertible Preferred Stock Liquidation Carrying Original Issue Price Authorized Outstanding Value Value Per Share Series A redeemable convertible preferred stock 59,433,105 5,794,727 $ 47,178 $ 40,735 $ 8.1415 Series B redeemable convertible preferred stock 75,500,000 7,345,067 97,520 97,656 $ 13.2769 134,933,105 13,139,794 $ 144,698 $ 138,391 Following the closing of the IPO in May, all outstanding shares of the redeemable convertible preferred stock converted into 13,139,794 shares of common stock and the related carrying value was reclassified to common stock and additional paid-in capital. There were no shares of redeemable convertible preferred stock outstanding as of December 31, 2019. The holders of the redeemable convertible preferred stock had various rights and preferences as follows: Voting Rights The holders of redeemable convertible preferred stock were entitled to vote on all matters on which the common stockholders were entitled to vote. Each holder of redeemable convertible preferred stock was entitled to the number of votes equal to the number of shares of common stock into which the shares of redeemable convertible preferred stock held by such holder could be converted as of the record date. Holders of redeemable convertible preferred stock and common stock generally voted as a single class. Dividends The holders of redeemable convertible preferred stock were entitled to receive annual dividends when, as and if declared by the Company’s Board of Directors, prior to any preference to the common stock, at a rate of $0.6513 and $1.0626 per share on each outstanding share of Series A redeemable convertible preferred stock and Series B redeemable convertible preferred stock, respectively. All dividends to holders of preferred stock would have been paid on a pari passu Liquidation In the event of any liquidation, dissolution or winding up of the Company, a merger, acquisition or consolidation of the Company, any transaction or series of transactions in which more than 50% of the voting power of the Company had been transferred, or a sale, transfer, exclusive license or other disposition, in a single transaction or series of related transactions, of all or substantially all of the assets of the Company, the holders of redeemable convertible preferred stock would have been entitled to receive prior to and in preference to any distribution to holders of common stock, an amount equal to the original issue price, adjusted for any stock splits, stock dividends, recapitalizations, reclassifications, combinations or similar transactions (“anti-dilution adjustments”), plus all declared and unpaid dividends on such shares. Notwithstanding the above, for purposes of determining the amount each holder of shares of redeemable convertible preferred stock was entitled to receive with respect to a liquidation event, each such holder of shares of a series of redeemable convertible preferred stock would be deemed to have converted such holder’s shares of such series into shares of common stock immediately prior to the liquidation event if, as a result of an actual conversion, such holder would receive, in the aggregate, an amount greater than the amount that would be distributed to such holder if such holder did not convert such series of redeemable convertible preferred stock into shares of common stock. If any such holder would be deemed to have converted shares of redeemable convertible preferred stock into common stock pursuant to this paragraph, then such holder would be entitled to receive any distribution that would otherwise be made to holders of redeemable convertible preferred stock that had not converted into shares of common stock. Conversion Each share of redeemable convertible preferred stock was convertible, at the option of the holder, into the number of fully-paid and non-assessable shares of common stock that resulted from dividing the applicable original issue price per share by the applicable conversion price per share at the time of conversion, as adjusted for any anti-dilution adjustments. If, after the issuance date of the redeemable convertible preferred stock, the Company issued or sold, or was deemed to have sold, additional shares of common stock at a price lower than the original issuance price, except for certain exceptions allowed, the conversion price of the redeemable convertible preferred stock would have been adjusted. As of December 31, 2018, the Company’s redeemable convertible preferred stock was convertible into the Company’s shares of common stock on a one-for-one basis. Each share of redeemable convertible preferred stock was convertible into common stock automatically immediately upon (i) the Company’s receipt of a written request for such conversion from the holders of 65% of the then outstanding shares of redeemable convertible preferred stock on an as-converted to common stock basis, or (ii) the consummation of an underwritten public offering of common stock pursuant to the Securities Act of 1933, in which the aggregate gross proceeds to the Company were not less than $50.0 million (before deductions of underwriters discounts and commissions) (“Qualified IPO”). Balance Sheet Classification The redeemable convertible preferred stock was recorded outside of permanent equity because while it was not mandatorily redeemable, it would have become redeemable upon the occurrence of certain deemed liquidation events that were considered not solely within the Company’s control. Accordingly, the redeemable convertible preferred stock was presented in the mezzanine section of the balance sheet.

Redeemable Convertible Prefer_2

Redeemable Convertible Preferred Stock Liability	12 Months Ended
	Dec. 31, 2019
Temporary Equity Disclosure [Abstract]
Redeemable Convertible Preferred Stock Liability	8. Redeemable Convertible Preferred Stock Liability In March 2016, the Company executed a Series A Preferred Stock Purchase Agreement to sell shares of Series A redeemable convertible preferred stock. The Series A redeemable convertible preferred stock issuance was structured in three tranches: (i) $17.4 million for 2,257,697 shares (the “First Tranche”), including 626,787 shares of Series A redeemable convertible preferred stock to convert a $4.1 million balance in convertible promissory notes at $6.5132 per share, (ii) $17.3 million for 2,122,219 shares on milestone achievement (the “Second Tranche Option”) and (iii) $11.5 million or 1,414,811 shares on milestone achievement (the “Third Tranche Option”). In March 2016, the Company recognized a redeemable convertible preferred stock liability as investors received the right to purchase from the Company, on the same terms, additional shares of Series A redeemable convertible preferred stock, in future tranches on achievement of milestones. As the Series A investors hold a majority of the board seats, the decision to complete the Second Tranche and Third Tranche Options was deemed to be outside of the control of the Company. The redeemable convertible preferred stock liability was valued using an option pricing model (“OPM”), which resulted in an initial fair value of $4.8 million and $4.8 million for the Company’s obligation to sell the redeemable convertible preferred stock related to the Second and Third Tranche Options, respectively. On December 31, 2016, the redeemable convertible preferred stock liability was revalued to $2.2 million and $3.5 million for the Second and Third Tranche Options, respectively. On February 15, 2017, the Company issued an additional 2,122,219 shares of Series A redeemable convertible preferred stock at $8.1415 per share thereby extinguishing the redeemable convertible preferred stock liability for the Second Tranche Option. Immediately prior to the closing of the Second Tranche, the Company remeasured the redeemable convertible preferred stock liability to its then fair value and recorded a gain of $2.1 million in other income (expense), net. Upon extinguishment of the redeemable convertible preferred stock liability for the Second Tranche Option, its fair value of $0.1 million was reclassified to redeemable convertible preferred stock on the balance sheet. As of December 31, 2017, the Company remeasured the redeemable convertible preferred stock liability for the Third Tranche Option to its fair value and recorded a gain of $0.3 million in other income (expense), net. As of December 31, 2017, the Company had a redeemable convertible preferred stock liability of $3.2 million on its balance sheet for the Third Tranche Option representing its fair value. On January 22, 2018, the Company issued 1,414,811 shares of Series A redeemable convertible preferred stock at $8.1415 per share for gross proceeds of $11.5 million, thereby extinguishing the Third Tranche Option redeemable convertible preferred stock liability. Immediately prior to the closing of the Third Tranche, the Company remeasured the redeemable convertible preferred stock liability to its then fair value and recorded a gain of $0.1 million in other income (expense), net. Upon extinguishment of the redeemable convertible preferred stock liability for the Third Tranche Option, its fair value of $3.1 million was reclassified to redeemable convertible preferred stock on the balance sheet. The redeemable convertible preferred stock liability for the Third Tranche Option was valued using the following assumptions under the OPM: January 22, 2018 Stock price 1.01 Expected term (years) — Expected volatility 0 % Risk-free interest rate 0.00 % Dividend yield 0 %

Common Stock

Common Stock	12 Months Ended
	Dec. 31, 2019
Stockholders Equity Note [Abstract]
Common Stock	9. Common Stock As of December 31, 2019 and December 31, December 31, The Company had reserved common stock, on an as-converted basis, for future issuance as follows: As of December 31, 2019 2018 Conversion of Series A redeemable convertible preferred stock — 5,794,727 Conversion of Series B redeemable convertible preferred stock — 7,345,067 Exercise of outstanding options under the 2015 and 2019 Plans 1,962,332 1,598,864 Issuance of common stock options under the 2015 Plan — 535,691 Issuance of common stock options under the 2019 Plan 1,036,746 — Issuance of common stock options under the Employee Stock Purchase Plan 195,000 — Total 3,194,078 15,274,349

Stock Option Plan

Stock Option Plan	12 Months Ended
	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock Option Plan	10. Stock Option Plan 2019 Incentive Award Plan The Company’s board of directors adopted and the Company’s stockholders approved, effective on the day immediately prior to the effectiveness of the registration statement on Form S-1, the 2019 Incentive Award Plan (the “2019 Plan”), under which the Company may grant cash and equity-based incentive awards to the Company’s employees, consultants and directors. Following the effectiveness of the 2019 Plan, the Company will not make any further grants under the 2015 Equity Incentive Plan (the “2015 Plan”). However, the 2015 Plan will continue to govern the terms and conditions of the outstanding awards granted under it. Shares of common stock subject to awards granted under the 2015 Plan that are forfeited or lapse unexercised and which following the effective date of the 2019 Plan are not issued under the 2015 Plan will be available for issuance under the 2019 Plan. Options granted under the 2019 Plan may be either incentive stock options (“ISOs”) or nonqualified stock options (“NSOs”). ISOs may be granted only to Company employees (including officers and directors who are also employees). NSOs may be granted to Company employees and consultants. 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, respectively. The exercise price of an ISO granted to an employee who, at the time of grant, owns stock representing more than 10% of the voting power of all classes of stock of the Company (a “10% stockholder”) shall be no less than 110% of the estimated fair value of the shares on the date of grant. To date, options granted under the 2019 Plan have a term of 10 years (or five years if granted to a 10% stockholder) and generally vest over a 4-year period with 1-year cliff vesting. 2015 Equity Incentive Plan In 2015, the Company established its 2015 Plan which provides for the granting of stock options to employees and consultants of the Company. Options granted under the 2015 Plan may be either ISOs or NSOs. 2019 Employee Stock Purchase Plan The Company’s board of directors adopted and the Company’s stockholders approved, effective on the day immediately prior to the effectiveness of the registration statement on Form S-1, the 2019 Employee Stock Purchase Plan (the “ESPP”). The ESPP provides eligible employees with the opportunity to acquire an ownership interest in the Company through periodic payroll deductions (at the rate of 1-15% of eligible compensation), over a six-month offering period, pursuant to the ESPP the offering period is determined by the Company in its discretion but may not exceed 27 months, at a price equal to the lesser of 85% of the fair market value of the common stock at either the first business day or last business day of the relevant offering period, provided that no more than 4,000 shares of common stock may be purchased by any one employee during each offering period. The ESPP is intended to constitute an “employee stock purchase plan” under Section 423(b) of the Internal Revenue Code of 1986, as amended. A total of 195,000 shares of common stock were initially reserved for issuance under the ESPP, subject to an annual increase on January 1 of each year, beginning on January 1, 2020. For the year ended December 31, 2019, the Company recorded less than $0.1 million of compensation expense related to participation in the ESPP. Stock-Based Compensation Expense Total stock-based compensation expense recorded related to awards granted to employees and non-employees was as follows (in thousands): Year Ended December 31, 2019 2018 2017 Research and development $ 956 $ 410 $ 80 General and administrative 1,212 540 56 Total stock-based compensation expense $ 2,168 $ 950 $ 136 Stock Options Activity under the Company’s stock option plan is set forth below: Outstanding Options Shares available for Grant Shares Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term (Years) Balance, January 1, 2019 535,691 1,598,864 $ 4.13 9.22 Additional shares authorized 978,500 — Options granted (667,067 ) 667,067 $ 9.66 Options exercised — (137,675 ) $ 2.24 Options repurchased 23,698 — $ 0.86 Options canceled 165,924 (165,924 ) $ 5.41 Balance, December 31, 2019 1,036,746 1,962,332 $ 6.03 8.63 Exercisable as of December 31, 2019 580,036 $ 4.28 8.21 Vested and expected to vest as of December 31, 2019 1,962,332 $ 6.03 8.63 The weighted-average grant-date fair value of options granted during the years ended December 31, 2019 and December 31, 2018 was $6.73 and $3.28 per share, respectively. The aggregate intrinsic value of options exercised for the years ended December 31, 2019 and December 31, 2018 was $0.8 million and $0.1 million, respectively. Intrinsic values are calculated as the difference between the exercise price of the underlying options and the fair value of the common stock on the date of exercise. As of December 31, 2019, the total unrecognized stock-based compensation expense for stock options was $5.9 million, which is expected to be recognized over a weighted-average period of 2.81 years. The total fair value of options vested for the years ended December 31, 2019 and December 31, 2018 was $2.0 million and $0.3 million, respectively. Early Exercise of Stock Options The terms of the 2015 Plan permit the exercise of options granted under the 2015 Plan prior to vesting, subject to required approvals. The shares so acquired prior to vesting are subject to a lapsing repurchase right in favor of the Company, exercisable upon a termination of the holder’s service with the Company prior to full vesting at the original purchase price of such shares. The proceeds are initially recorded in other liabilities from the early exercise of stock options and are reclassified to additional paid-in capital as the Company’s repurchase right lapses. During the years ended December 31, 2019 and December 31, 2018, the Company repurchased 23,698 and 17,900 shares of common stock, respectively. As of December 31, 2019 and December 31, 2018, shares that were subject to repurchase were 84,964 and 237,853, respectively. The aggregate exercise price of early exercised shares as of December 31, 2019 and December 31, 2018 was $0.1 million and $0.2 million, respectively, which were recorded in other current liabilities and other non-current liabilities. Stock - During the years ended December 31, 2019 and December 31, The fair values of options were calculated using the assumptions set forth below: Year Ended December 31, 2019 2018 2017 Expected term 5.4 - 6.1 years 5.8 - 6.1 years 5.9 - 6.1 years Expected volatility 77.2% - 85.5% 80.3% - 82.3% 79.4% - 92.8% Risk-free interest rate 1.4% - 2.5 % 2.6% - 3.1% 1.9% - 2.3% Dividend yield 0% 0% 0% Expected term . The expected term represents the weighted-average period the stock options are expected to remain outstanding and is based on the options’ vesting terms, contractual terms and industry peers, as the Company does not have sufficient historical information to develop reasonable expectations about future exercise patterns and post-vesting employment termination behavior. Expected Volatility . The Company uses an average historical stock price volatility of a peer group of publicly traded companies to be representative of its expected future stock price volatility, as the Company does not have sufficient trading history for its common stock. For purposes of identifying these peer companies, the Company considers the industry, stage of development, size and financial leverage of potential comparable companies. For each grant, the Company measures historical volatility over a period equivalent to the expected term. 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 risk-free rate assumption is based on U.S. Treasury instruments whose term was consistent with the expected term of the Company’s stock options. Expected Dividend Rate . The Company has not paid and does not anticipate paying any dividends in the near future. Accordingly, the Company has estimated the dividend yield to be zero. The Company accounts for forfeitures as they occur. Fair Value of Common Stock Prior to the IPO, the fair value of the Company’s common stock was determined by the board of directors with assistance from management and, in part, on input from an independent third-party valuation firm. The board of directors determined the fair value of common stock by considering a number of objective and subjective factors, including valuations of comparable companies, sales of redeemable convertible preferred stock, operating and financial performance, the lack of liquidity of the Company’s common stock and the general and industry-specific economic outlook. Subsequent to the Company’s IPO, the fair value of the Company’s common stock is determined based on its closing market price on the date of grant on the NASDSQ Global Select Market. Stock - During the years ended December 31, 2019 and December 31, 2018 and 7,800 , respectively Restricted Stock Restricted stock activity was as follows: Number of Shares of Restricted Stock Outstanding Weighted Average Grant Date Fair Value Unvested, December 31, 2018 19,492 $ 0.63 Vested (4,867 ) $ 0.05 Unvested, December 31, 2019 14,625 $ 0.82 During the years ended December 31, 2015 and 2016, the Company issued 346,579 shares of restricted stock to its founders under founder stock purchase agreements. Under the terms of these agreements, the restricted stock vests either over the requisite service periods, or the achievement of pre-defined business development milestones. Holders of restricted stock have voting and dividend rights with respect to such shares held without regard to vesting. Shares of restricted stock are subject to a right of repurchase at the original purchase price held by the Company. As the restricted stock was purchased by employees at a price equal to its fair value at the time of issuance, there was no stock-based compensation expense related to these awards. The total fair value of restricted stock vested during the years ended December 31, 2019 and December 31, 2018, was $0.1 million and less than $0.1 million, respectively. As of December 31, 2019 and December 31, 2018, 14,625 and 19,492 shares of restricted stock, respectively, were outstanding and subject to a right of repurchase, with an aggregate purchase price of less than $0.1 million, which is recorded in other current liabilities and other non-current liabilities on the balance sheets.

Net Loss Per Share Attributable

Net Loss Per Share Attributable to Common Stockholders	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Net Loss Per Share Attributable to Common Stockholders	11. Net Loss Per Share Attributable to Common Stockholders The following table sets forth the computation of basic and diluted net loss per share attributable to common stockholders (in thousands, except share and per share data): Year Ended December 31, 2019 2018 2017 Numerator: Net loss attributable to common stockholders $ (41,975 ) $ (34,346 ) $ (11,862 ) Denominator: Weighted-average shares outstanding 12,669,367 1,338,776 1,181,179 Less: weighted-average shares of restricted stock that are subject to repurchase (172,410 ) (382,524 ) (485,807 ) Weighted-average shares used in computing net loss per share attributable to common stock, basic and diluted 12,496,957 956,252 695,372 Net loss per share attributable to common stockholders, basic and diluted $ (3.36 ) $ (35.92 ) $ (17.06 ) The following outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods presented because including them would have been antidilutive: As of December 31, 2019 2018 2018 Redeemable convertible preferred stock — 13,139,794 4,379,916 Options to purchase common stock 1,962,332 1,598,864 388,966 Restricted stock 14,625 19,492 65,510 Restricted stock acquired upon early exercise of stock options 84,964 237,853 397,993 Total 2,061,921 14,996,003 5,232,385 As of December 31, 2019, the Company has contributions from plan participant of less than $0.1 million under the ESPP, which if converted, would be equivalent to 4,378 shares based on 85% of the stock price at the beginning of the offering period.

401(k) Retirement Savings Plan

401(k) Retirement Savings Plan	12 Months Ended
	Dec. 31, 2019
Compensation And Retirement Disclosure [Abstract]
401(k) Retirement Savings Plan	12. 401(k) Retirement Savings Plan In 2016, the Company adopted a 401(k) plan for all employees who have met certain eligibility requirements. Under the agreement, employees may elect to contribute up to 90% of their eligible compensation to a 401(k) plan, subject to certain limitations. The Company is responsible for administrative costs of the 401(k) plan. The Company may, at its discretion, make matching or profit sharing contributions to the 401(k) plan For the year ended December 31, 2019, , the Company made matching contributions of $0.2 million,

License Agreements

License Agreements	12 Months Ended
	Dec. 31, 2019
License Agreements [Abstract]
License Agreements	13. License Agreements Novartis International Pharmaceuticals Ltd. (“Novartis”) In September 2018, the Company entered into a license agreement with Novartis International Pharmaceuticals Ltd. (“Novartis”) to develop and commercialize Novartis’ LXS196 (also known as IDE196), a Phase 1 protein kinase C (“PKC”) inhibitor for the treatment of cancers having GNAQ and GNA11 mutations. In consideration of license and rights granted under the license agreement, the Company made a one-time cash payment of $2.5 million to Novartis and issued 263,615 shares of Series B redeemable convertible preferred stock with a fair value of $3.8 million to an affiliate of Novartis, which were recorded within research and development expenses on the Company’s statements of operations and comprehensive loss, as the products have not reached technological feasibility and do not have alternative future use. Under the license agreement, the Company is liable to make contingent development and sales milestone payments of up to $29.0 million and mid to high single digit royalty payments of the net sales of licensed products.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2019
Related Party Transactions [Abstract]
Related Party Transactions	14. Related Party Transactions The Company incurred $1.3 million and $0.3 million of research and development expenses during the years ended December 31, 2018 and December 31, 2017, respectively, in relation to agreements for pharmaceutical research, development and manufacturing activities with entities affiliated with a former director of the Company, Edward Hu. As of December 31, 2018, $0.2 million is included in accounts payable in the Company’s balance sheets. Mr. Hu resigned from the Company’s board of directors in January 2019.

Supplementary Financial Data (U

Supplementary Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Supplementary Financial Data (Unaudited)	15. Supplementary Financial Data (Unaudited) The following table presents the supplementary quarterly financial data for the years ended December 31, 2019 and December 31, 2018. Three Months Ended March 31 June 30 September 30 (1) December 31 2019 Total operating expenses $ 10,094 $ 11,235 $ 11,623 (1) $ 11,319 Net loss $ (9,569 ) $ (10,656 ) $ (10,969 ) $ (10,781 ) Net loss attributable to common stockholders $ (9,569 ) $ (10,656 ) $ (10,969 ) $ (10,781 ) Net loss per share attributable to common stockholders, basic and diluted $ (8.69 ) $ (1.30 ) $ (0.54 ) $ (0.53 ) 2018 Total operating expenses $ 6,087 $ 7,571 $ 13,626 $ 9,133 Net loss $ (5,737 ) $ (6,995 ) $ (13,052 ) $ (8,562 ) Net loss attributable to common stockholders $ (5,737 ) $ (6,995 ) $ (13,052 ) $ (8,562 ) Net loss per share attributable to common stockholders, basic and diluted $ (6.91 ) $ (7.47 ) $ (13.06 ) $ (8.11 ) (1) Results for the quarter ended September 30, 2018 included a $6.3 million expense related to the Novartis license fee (as further described in Note 13 to the financial statements).

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The financial statements and accompanying notes have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”).
Reverse Stock Split	Reverse Stock Split In May 2019, the Company’s board of directors approved a 1-for-10.2564 reverse stock split of the Company’s common stock and redeemable convertible preferred stock, which was effected on May 21, 2019. The par value and authorized shares of the common stock and redeemable convertible preferred stock were not adjusted as a result of the reverse stock split. All issued and outstanding common stock, options to purchase common stock and per share amounts contained in these financial statements have been retroactively adjusted to give effect to the reverse stock split for all periods presented.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of expenses during the reporting period. Such estimates include useful lives of property and equipment, determination of the discount rate for operating leases, accruals for research and development activities, stock-based compensation, income taxes, and prior to the IPO, the valuation of the redeemable convertible preferred stock liability. Actual results could differ from those estimates.
Segments	Segments The Company operates and manages its business as one operating and reportable segment, which is the business of research and development for oncology-focused precision medicine. The Company’s chief executive officer, who is the chief operating decision maker, reviews financial information on an aggregate basis for purposes of allocating resources and evaluating financial performance. All of the Company's long-lived assets are located in the United States.
Risks and Uncertainties	Risks and Uncertainties The Company operates in a dynamic and highly competitive industry and is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, protection of proprietary technology, dependence on key personnel, contract manufacturer and contract research organizations, compliance with government regulations and the need to obtain additional financing to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical studies and clinical trials and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance and reporting. The Company believes that changes in any of the following areas could have a material adverse effect on the Company’s future financial position, results of operations, or cash flows; ability to obtain future financing; advances and trends in new technologies and industry standards; results of clinical trials; regulatory approval and market acceptance of the Company’s products; development of sales channels; certain strategic relationships; litigation or claims against the Company base on intellectual property, patent, product, regulatory, or other factors; and the Company’s ability to attract and retain employees necessary to support its growth. Products developed by the Company require approvals from the U.S. Food and Drug Administration (“FDA”) or other international regulatory agencies prior to commercial sales. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained or maintained, that the products will receive the necessary approvals, or that any approved products will be commercially viable. If the Company was denied approval, approval was delayed or the Company was unable to maintain approval, it could have a materially adverse impact on the Company. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from other pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees, consultants and other third parties. In December 2019, a novel strain of coronavirus was reported in Wuhan, China. The extent of the impact of the coronavirus outbreak on the Company’s business will depend on certain developments, including the duration and spread of the outbreak and the extent and severity of the impact on the Company’s clinical trial activities and suppliers, all of which are uncertain and cannot be predicted. At this point, the extent to which the coronavirus outbreak may materially impact the Company’s financial condition, liquidity or results of operations is uncertain. The Company has expended and will continue to expend substantial funds to complete the research, development and clinical testing of product candidates. The Company also will be required to expend additional funds to establish commercial-scale manufacturing arrangements and to provide for the marketing and distribution of products that receive regulatory approval. The Company may require additional funds to commercialize its products. The Company is unable to entirely fund these efforts with its current financial resources. If adequate funds are unavailable on a timely basis from operations or additional sources of financing, the Company may have to delay, reduce the scope of or eliminate one or more of its research or development programs which would materially and adversely affect its business, financial condition and operations.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash, cash equivalents and marketable securities. Substantially all the Company’s cash is held by one financial institution that management believes is of high credit quality. Such deposits may, at times, exceed federally insured limits. The Company’s investment policy addresses credit ratings, diversification, and maturity dates. The Company invests its cash equivalents and marketable securities in money market funds, U.S. government securities, commercial paper, corporate bonds, and asset-backed securities. The Company limits its credit risk associated with cash equivalents and marketable securities by placing them with banks and institutions it believes are highly creditworthy and in highly rated investments and, by policy, limits the amount of credit exposure with any one commercial issuer. The Company has not experienced any credit losses on its deposits of cash, cash equivalents or marketable securities.
Deferred Initial Public Offering Costs	Deferred Initial Public Offering Costs Deferred offering costs, consisting of legal, accounting and other fees and costs relating to the Company’s IPO, are capitalized and recorded on the balance sheet. As of December 31, 2018, $0.6 million of deferred offering costs were recorded on the balance sheet. During the year ended December 31, 2019, an additional $2.7 million in deferred offering costs were incurred. In connection with the IPO, all deferred offering costs reduced the gross proceeds from the IPO and were recorded within additional paid-in capital on the accompanying balance sheet as of December 31, 2019.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash equivalents that are readily convertible to cash are stated at cost, which approximates fair value. The Company considers all highly liquid investments purchased with an original or remaining maturity of three months or less at the date of purchase to be cash equivalents.
Restricted Cash	Restricted Cash Restricted cash as of December 31, 2019 and December 31, 2018 consisted of cash balances held as security in connection with the Company’s facility lease agreement in South San Francisco, California. The balances are classified as long-term assets on the Company’s balance sheet.
Marketable Securities	Marketable Securities Marketable securities are investments in marketable securities with maturities greater than three months at the time of purchase. The Company determines the appropriate classification of its investments in marketable securities at the time of purchase and reevaluates such designation at each balance sheet date. The Company has classified and accounted for its marketable securities as available-for-sale. After consideration of the Company’s risk versus reward objectives and liquidity requirements, the Company may sell these securities prior to their stated maturities. The Company classifies highly liquid securities with maturities beyond 12 months as long-term marketable securities in the balance sheet. These securities are carried at fair value as determined based upon quoted market prices or pricing models for similar securities. Unrealized gains and losses, if any, are excluded from earnings and are reported as a component of accumulated other comprehensive income (loss). 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, if any, on available-for-sale securities are included in other income (expense), net. The cost of securities sold is based on the specific identification method. Interest and dividends on securities classified as available-for-sale are included in interest income. The Company did not identify any of its marketable securities as other-than-temporarily impaired as of December 31, 2019 .
Fair Value of Financial Instruments	Fair Value of Financial Instruments The carrying amounts of the Company’s certain financial instruments, including cash equivalents, accounts payable and accrued liabilities approximate fair value due to their relatively short maturities and market interest rates if applicable. The carrying amount of the redeemable convertible preferred stock liability represents its fair value. Refer to Note 3 for details on the fair value of marketable securities.
Property and Equipment, Net	Property and Equipment, Net Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation is computed using the straight-line method over the estimated useful lives of the assets, which is generally between three and five years. Leasehold improvements are stated at cost and amortized over the shorter of the useful lives of the assets or the lease term
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company reviews property and equipment for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset or asset group may not be recoverable. Recoverability is measured by comparison of the carrying amount of the asset or asset group to the future net cash flows which the asset or asset group is expected to generate. If such asset or asset group is considered to be impaired, the impairment to be recognized is measured by the amount by which the carrying amount of the asset or asset group exceeds the fair value of the asset or asset group. There have been no such impairments of long-lived assets for the years ended December 31, 2019 and December 31, 2018.
Leases	Leases The Company leases its facilities and meets the requirements to account for these leases as operating leases. For the year ended December 31, 2018, for facility leases that contain rent escalations or rent concession provisions, the Company records its lease expense during the lease term on a straight-line basis over the term of the lease. As of December 31, 2018, the Company recorded the difference between the rent paid and the straight-line rent as a deferred rent liability. As of December 31, 2018, leasehold improvements funded by landlord incentives or allowances were recorded as leasehold improvement assets and a corresponding deferred rent liability. The leasehold improvement asset is amortized over the lesser of the term of the lease or life of the asset. For the year ended December 31, 2018, the deferred rent liability is amortized on a straight-line basis as a reduction to rent expense over the term of the lease agreement. Upon adoption of ASC 842, Leases . ROU assets and lease liabilities are recognized based on the present value of the future lease payments over the lease term at commencement date. As most of the Company’s leases do not provide an implicit rate, the Company uses its incremental borrowing rate based on the information available at commencement date in determining the present value of future payments. The ROU asset also includes any lease payments made to the lessor at or before the commencement date, minus lease incentives received, and initial direct costs incurred. The Company’s lease terms may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Lease expense for lease payments is recognized on a straight-line basis over the lease term. For lease agreements entered into or reassessed after the adoption of ASC 842, the Company combines lease and nonlease components.
Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock The Company recorded all shares of redeemable convertible preferred stock at their respective fair values on the dates of issuance, net of issuance costs. The redeemable convertible preferred stock was recorded outside of permanent equity because while it was not mandatorily redeemable, in certain events considered not solely within the Company’s control, such as a merger, acquisition, or sale of all or substantially all of the Company’s assets (each, a “deemed liquidation event”), the convertible preferred stock would become redeemable at the option of the holders of at least a majority of the then outstanding such shares. The Company did not adjust the carrying values of the redeemable convertible preferred stock to its liquidation preference because a deemed liquidation event obligating the Company to pay the liquidation preferences to holders of shares of redeemable convertible preferred stock was not probable of occurring. Subsequent adjustments to the carrying values to the liquidation preferences would have been made only when it became probable that such a deemed liquidation event would occur.
Redeemable Convertible Preferred Stock Liability	Redeemable Convertible Preferred Stock Liability The obligation to issue additional shares of Series A redeemable convertible preferred stock at future dates pursuant to a preferred stock purchase agreement was determined to be a freestanding instrument that should be accounted for as a liability. At initial recognition, the Company recorded the redeemable convertible preferred stock liability on the balance sheet at its fair value. The liability was until it was extinguished upon issuance of Series A redeemable convertible preferred stock in January 2018.
Research and Development Expenses	Research and Development Expenses Research and development expenses consist of compensation costs, employee benefit costs, costs for contract manufacturing organizations (“CMOs”), costs for contract research organizations (“CROs”), costs for sponsored research, consulting costs, costs for laboratory supplies, costs for product licenses, facility-related expenses and depreciation. All research and development costs are charged to research and development expenses within the statements of operations and comprehensive loss as incurred. Payments associated with licensing agreements to acquire exclusive licenses to develop, use, manufacture and commercialize products that have not reached technological feasibility and do not have alternate commercial use are also expensed as incurred. Payments made to third parties under these arrangements in advance of the performance of the related services by the third parties are recorded as prepaid expenses until the services are rendered.
Accrued Research and Development	Accrued Research and Development The Company has entered into various agreements with CMOs and CROs. The Company’s research and development accruals are estimated based on the level of services performed, progress of the studies, including the phase or completion of events, and contracted costs. The estimated costs of research and development provided, but not yet invoiced, are included in accrued liabilities on the balance sheet. If the actual timing of the performance of services or the level of effort varies from the original estimates, the Company will adjust the accrual accordingly. Payments made to CMOs and CROs under these arrangements in advance of the performance of the related services are recorded as prepaid expenses and other current assets until the services are rendered.
Stock-Based Compensation	Stock-Based Compensation The Company accounts for stock-based compensation arrangements with employees in accordance with ASC 718, Stock Compensation. For the years ended December 31, 2018 and December 31, 2017, the Company accounted for stock options issued to non-employees based on the estimated fair value of the awards using the Black-Scholes option pricing model in accordance with ASC 505-50, Equity-Based Payment to Non-employees Upon adoption of ASU 2018-07, Compensation–Stock Compensation (Topic 718): Improvements to Non-employee Share–Based Payment Accounting ASC 718, Stock Compensation.
Income Taxes	Income Taxes The Company accounts for income taxes using the asset and liability method whereby deferred tax assets and liabilities are determined based on differences between financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that are currently in effect unless such rate is expected to be different when the deferred item reverses. Valuation allowances are established where necessary to reduce deferred tax assets to the amounts expected to be realized. Deferred tax assets and liabilities are classified as noncurrent on the balance sheet. The Company recognizes the effect of income tax positions only if those positions are more likely than not of being sustained. Recognized income tax positions are measured at the largest amount that has a greater than 50% likelihood of being realized. Changes in recognition or measurement are reflected in the period in which the change in judgment occurs. The Company records interest and penalties related to unrecognized tax benefits in interest expense and other expense, respectively.
Comprehensive Loss	Comprehensive Loss Comprehensive loss represents all changes in stockholders’ equity (deficit) except those resulting from and distributions to stockholders.
Net Loss per Share Attributable to Common Stockholders	Net Loss per Share Attributable to Common Stockholders Basic net loss per common share is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of common stock outstanding during the period, without consideration of potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common stock and potentially dilutive securities outstanding for the period. For purposes of the diluted net loss per share calculation, redeemable convertible preferred stock, stock options and restricted stock that is subject to repurchase at the original purchase price are considered to be potentially dilutive securities. Basic and diluted net loss attributable to common stockholders per share is presented in conformity with the two-class method required for participating securities as the redeemable convertible preferred stock is considered a participating security because it participates in dividends with common stock. The Company also considers the shares issued upon the early exercise of stock options subject to repurchase to be participating securities, because holders of such shares have non-forfeitable dividend rights in the event a dividend is paid on common stock. The holders of all series of redeemable convertible preferred stock and the holders of early exercised shares subject to repurchase do not have a contractual obligation to share in the Company’s losses. As such, the net loss was attributed entirely to common stockholders. Because the Company has reported a net loss for all periods presented, diluted net loss per common share is the same as basic net loss per common share for those periods.
Recent Accounting Pronouncements	Recent Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (“FASB”) under its accounting standard codifications (“ASC”) or other standard setting bodies and adopted by the Company as of the specified effective date, unless otherwise discussed below. Recently Adopted Accounting Pronouncements In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) Leases Codification Improvements to Topic 842, Leases Leases (Topic 842): Targeted Improvements Leases (Topic 842): Codification Improvements. for its operating leases as of January 1, 2019, including deferred rent of $1.9 million The adoption of these ASUs did not have any impact on the statements of operations and comprehensive loss and statements of cash flows. See Note 5 for more information related to the Company’s lease obligations. In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260); Distinguishing Liabilities from Equity (Topic 480); Derivatives and Hedging (Topic 815): (Part I) Accounting for Certain Financial Instruments with Down Round Features, (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception In February 2018, the FASB issued ASU 2018-02, Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Non-employee Share-Based Payment Accounting New Accounting Pronouncements Not Yet Adopted In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which requires the measurement and recognition of expected credit losses for financial assets held at amortized cost. ASU 2016-13 replaces the existing incurred loss impairment model with an expected loss model. It also eliminates the concept of other-than-temporary impairment and requires credit losses related to available-for-sale debt securities to be recorded through an allowance for credit losses rather than as a reduction in the amortized cost basis of the securities. These changes will result in earlier recognition of credit losses. For public business entities, this ASU is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. The FASB subsequently issued supplemental guidance to ASC 326 within ASU 2019-05, Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief , ASU 2019-10, Financial Instruments—Credit Losses (Topic 326), Derivatives and Hedging (Topic 815), and Leases (Topic 842) and ASU 2019-11, Codification Improvements to Topic 326, Financial Instruments—Credit Losses . ASU 2019-05 provides an option to irrevocably elect the fair value option for certain financial assets previously measured at amortized cost basis. ASU 2019-10 extended the effectiveness of Topic 326 for smaller reporting companies until fiscal years beginning after December 31, 2020. Early adoption is permitted. The Company is currently evaluating the impact the adoption of these ASUs will have on its financial statements and related disclosures, and the adoption timing of these ASUs In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement , which modifies the disclosure requirements on fair value measurements. ASU 2018-13 removes the requirement to disclose: the amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy; the policy for timing of transfers between levels; and the valuation processes for Level 3 fair value measurements. For public business entities, this ASU is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. The Company is currently evaluating the impact the adoption of this ASU will have on its financial statements and related disclosures. In August 2018, the FASB issued ASU 2018-15, Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. ASU 2018-15 requires that certain implementation costs incurred in a cloud computing arrangement be deferred and recognized over the term of the arrangement. For public business entities, this ASU is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. The Company is currently evaluating the impact the adoption of this ASU will have on its financial statements and related disclosures. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. ASU 2019-12 simplifies the accounting for income taxes by removing certain exceptions to the general principles in Topic 740 and improves consistent application of and simplify GAAP for other areas of Topic 740 by clarifying existing guidance. For public business entities, this ASU is effective for fiscal years beginning after December 15, 2020, and interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the impact the adoption of this ASU will have on its financial statements and related disclosures.
Fair Value Measurement	The Company applies fair value accounting for all financial assets and liabilities and non-financial assets and liabilities that are recognized or disclosed at fair value in the financial statements on a recurring basis. Fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, a three-tier fair value hierarchy has been established, which prioritizes the inputs used in measuring fair value as follows: Level 1—Observable inputs, such as quoted prices in active markets for identical assets or liabilities at the measurement date . Level 2—Observable inputs other than Level 1 prices such as quoted prices for similar assets or liabilities, quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities . Level 3—Unobservable inputs which reflect management’s best estimate of what market participants would use in pricing the asset or liability at the measurement date. Consideration is given to the risk inherent in the valuation technique and the risk inherent in the inputs to the model . In determining fair value, the Company utilizes valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible as well as considers counterparty credit risk in its assessment of fair value. The Company’s Level 3 liabilities consist of the redeemable convertible preferred stock liability. The determination of the fair value of the redeemable convertible preferred stock liability is discussed in Note 8.
Balance Sheet Classification	Balance Sheet Classification The redeemable convertible preferred stock was recorded outside of permanent equity because while it was not mandatorily redeemable, it would have become redeemable upon the occurrence of certain deemed liquidation events that were considered not solely within the Company’s control. Accordingly, the redeemable convertible preferred stock was presented in the mezzanine section of the balance sheet.
Early Exercise of Stock Options	Early Exercise of Stock Options
Share Based Compensation Forfeiture	The Company accounts for forfeitures as they occur.

Fair Value Measurement (Tables)

Fair Value Measurement (Tables)	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Schedule of Financial Assets and Liabilities Measured and Recognized at Fair Value	As of December 31, 2019, financial assets and liabilities measured and recognized at fair value are as follows (in thousands): December 31, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Assets U.S. government securities Level 1 $ 24,973 $ 32 $ — $ 25,005 Corporate bonds Level 2 39,185 37 (4 ) 39,218 Commercial paper Level 2 2,192 — — 2,192 Marketable securities 66,350 69 (4 ) 66,415 Money market funds ( 1) Level 1 34,008 — — 34,008 Total fair value of assets $ 100,358 $ 69 $ (4 ) $ 100,423 (1) Included in cash and cash equivalents on the balance sheet As of December 31, 2018, financial assets measured and recognized at fair value are as follows (in thousands): December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Assets U.S. government securities Level 1 $ 9,983 $ — $ (1 ) $ 9,982 Corporate bonds Level 2 18,820 — (19 ) 18,801 Commercial paper Level 2 29,699 — — 29,699 Asset-backed securities Level 2 10,984 — (10 ) 10,974 Marketable securities 69,486 — (30 ) 69,456 Money market funds ( 1) Level 1 20,419 — — 20,419 Total fair value of assets $ 89,905 $ — $ (30 ) $ 89,875 (1) Included in cash and cash equivalents on the balance sheet
Schedule of Changes in Fair value of Financial Liabilities	The following tables set forth the changes in the fair value of Level 3 financial liabilities (in thousands): Redeemable Convertible Preferred Stock Liability Fair value as of January 1, 2018 $ 3,207 Issuance of redeemable convertible preferred stock (3,137 ) Change in fair value included in other income (expense), net (70 ) Fair value as of December 31, 2018 $ —

Balance Sheet Components (Table

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2019
Balance Sheet Related Disclosures [Abstract]
Summary of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): Useful Life As of December 31, (In Years) 2019 2018 Laboratory equipment 5 $ 4,034 $ 3,335 Computer equipment 3 117 117 Software 3 118 109 Leasehold improvements Shorter of useful life or lease term 2,581 2,570 Furniture and fixtures 5 308 304 Total property and equipment 7,158 6,435 Less: Accumulated depreciation and amortization (2,516 ) (1,283 ) Property and equipment, net $ 4,642 $ 5,152
Summary of Accrued Liabilities	Accrued liabilities consisted of the following (in thousands): As of December 31, 2019 2018 Accrued salaries and benefits $ 1,733 $ 1,279 Accrued research and development expenses 2,787 1,095 Legal and professional fees 457 999 Other 46 210 Accrued liabilities $ 5,023 $ 3,583

Leases, Lease Commitments and_2

Leases, Lease Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2019
Leases [Abstract]
Schedule of Future Minimum Lease Payments Under Non-cancelable Operating Leases	Future minimum lease payments under the non-cancelable operating leases as of December 31, 2018 are as follows (in thousands): As of December 31, 2018: Operating Leases 2019 $ 1,711 2020 1,611 2021 1,604 2022 1,646 2023 1,690 Thereafter 1,441 Total lease payments $ 9,703
Schedule of Maturities of Operating Lease Liabilities	The maturities of operating lease liabilities as of December 31, 2019 are as follows (in thousands): As of December 31, 2019 Operating Leases 2020 $ 1,564 2021 1,604 2022 1,646 2023 1,690 2024 1,441 Total lease payments 7,945 Less: Interest (1,173 ) Present value of lease liabilities $ 6,772 Amounts recognized on the balance sheet Current lease liabilities $ 1,145 Long-term lease liabilities 5,627 Total lease liabilities $ 6,772

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Schedule of Provision for Income Taxes	The provision for income taxes differs from the amount expected by applying the federal statutory rate to the loss before taxes as follows: Year Ended December 31, 2019 2018 2017 Federal statutory income tax rate 21.0 % 21.0 % 34.0 % State income taxes 7.0 % 6.9 % 6.9 % Change in valuation allowance (31.4 %) (29.4 %) (29.2 %) Research tax credits 3.6 % 2.7 % 2.9 % Other permanent differences (0.4 %) (1.3 %) (2.1 %) Change in fair value of redeemable convertible preferred stock liability 0.2 % 0.1 % 7.0 % Remeasurement of deferred tax due to tax law change — % — % (19.5 %) Provision for income taxes 0.0 % 0.0 % 0.0 %
Schedule of Tax Effects of Temporary Differences and Carryforwards of Deferred Tax Assets	The tax effects of temporary differences and carryforwards of the deferred As of December 31, 2019 2018 Deferred tax assets: Net operating loss carryforwards $ 23,947 $ 12,633 Research and development credit carryforwards 2,995 1,458 Lease liability 1,895 — Intangible assets 1,809 1,913 Accruals and reserves 436 1,048 Stock-based compensation 407 — Gross deferred tax assets 31,489 17,052 Less: Valuation allowance (29,462 ) (16,289 ) Deferred tax assets, net of valuation allowance 2,027 763 Deferred tax liabilities: Right-of-use assets (1,415 ) — Property and equipment (612 ) (724 ) Stock-based compensation — (39 ) Net deferred tax assets $ — $ —
Summary of Activity Related to Company’s Unrecognized Tax Benefits	The following table summarizes the activity related to the Company’s unrecognized tax benefits (in thousands). Balance as of January 1, 2018 $ 174 Increase related to prior year tax positions 12 Increase related to current year tax positions 213 Balance as of December 31, 2018 $ 399 Increase related to prior year tax positions 59 Increase related to current year tax positions 288 Balance as of December 31, 2019 $ 746

Redeemable Convertible Prefer_3

Redeemable Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2019
Temporary Equity Disclosure [Abstract]
Schedule of Issued and Outstanding Redeemable Convertible Preferred Stock and Principal Terms	Issued and outstanding redeemable convertible preferred stock and its principal terms as of December 31, 2018 were as follows (in thousands, except share and per share amounts) Redeemable Convertible Preferred Stock Liquidation Carrying Original Issue Price Authorized Outstanding Value Value Per Share Series A redeemable convertible preferred stock 59,433,105 5,794,727 $ 47,178 $ 40,735 $ 8.1415 Series B redeemable convertible preferred stock 75,500,000 7,345,067 97,520 97,656 $ 13.2769 134,933,105 13,139,794 $ 144,698 $ 138,391

Redeemable Convertible Prefer_4

Redeemable Convertible Preferred Stock Liability (Tables)	12 Months Ended
	Dec. 31, 2019
Temporary Equity Disclosure [Abstract]
Schedule of Redeemable Convertible Preferred Stock Liability Valuation Assumptions under OPM	The redeemable convertible preferred stock liability for the Third Tranche Option was valued using the following assumptions under the OPM: January 22, 2018 Stock price 1.01 Expected term (years) — Expected volatility 0 % Risk-free interest rate 0.00 % Dividend yield 0 %

Common Stock (Tables)

Common Stock (Tables)	12 Months Ended
	Dec. 31, 2019
Stockholders Equity Note [Abstract]
Schedule of Number of Common Stock Reserved for Future Issuance	The Company had reserved common stock, on an as-converted basis, for future issuance as follows: As of December 31, 2019 2018 Conversion of Series A redeemable convertible preferred stock — 5,794,727 Conversion of Series B redeemable convertible preferred stock — 7,345,067 Exercise of outstanding options under the 2015 and 2019 Plans 1,962,332 1,598,864 Issuance of common stock options under the 2015 Plan — 535,691 Issuance of common stock options under the 2019 Plan 1,036,746 — Issuance of common stock options under the Employee Stock Purchase Plan 195,000 — Total 3,194,078 15,274,349

Stock Option Plan (Tables)

Stock Option Plan (Tables)	12 Months Ended
	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock-Based Compensation Expense	Total stock-based compensation expense recorded related to awards granted to employees and non-employees was as follows (in thousands): Year Ended December 31, 2019 2018 2017 Research and development $ 956 $ 410 $ 80 General and administrative 1,212 540 56 Total stock-based compensation expense $ 2,168 $ 950 $ 136
Summary of Activity under Stock Option Plan	Activity under the Company’s stock option plan is set forth below: Outstanding Options Shares available for Grant Shares Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term (Years) Balance, January 1, 2019 535,691 1,598,864 $ 4.13 9.22 Additional shares authorized 978,500 — Options granted (667,067 ) 667,067 $ 9.66 Options exercised — (137,675 ) $ 2.24 Options repurchased 23,698 — $ 0.86 Options canceled 165,924 (165,924 ) $ 5.41 Balance, December 31, 2019 1,036,746 1,962,332 $ 6.03 8.63 Exercisable as of December 31, 2019 580,036 $ 4.28 8.21 Vested and expected to vest as of December 31, 2019 1,962,332 $ 6.03 8.63
Assumptions Used to Calculate Fair Values of Options	The fair values of options were calculated using the assumptions set forth below: Year Ended December 31, 2019 2018 2017 Expected term 5.4 - 6.1 years 5.8 - 6.1 years 5.9 - 6.1 years Expected volatility 77.2% - 85.5% 80.3% - 82.3% 79.4% - 92.8% Risk-free interest rate 1.4% - 2.5 % 2.6% - 3.1% 1.9% - 2.3% Dividend yield 0% 0% 0%
Summary of Restricted Stock Activity	Restricted stock activity was as follows: Number of Shares of Restricted Stock Outstanding Weighted Average Grant Date Fair Value Unvested, December 31, 2018 19,492 $ 0.63 Vested (4,867 ) $ 0.05 Unvested, December 31, 2019 14,625 $ 0.82

Net Loss Per Share Attributab_2

Net Loss Per Share Attributable to Common Stockholders (Tables)	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders	The following table sets forth the computation of basic and diluted net loss per share attributable to common stockholders (in thousands, except share and per share data): Year Ended December 31, 2019 2018 2017 Numerator: Net loss attributable to common stockholders $ (41,975 ) $ (34,346 ) $ (11,862 ) Denominator: Weighted-average shares outstanding 12,669,367 1,338,776 1,181,179 Less: weighted-average shares of restricted stock that are subject to repurchase (172,410 ) (382,524 ) (485,807 ) Weighted-average shares used in computing net loss per share attributable to common stock, basic and diluted 12,496,957 956,252 695,372 Net loss per share attributable to common stockholders, basic and diluted $ (3.36 ) $ (35.92 ) $ (17.06 )
Schedule of Outstanding Shares of Potentially Dilutive Securities Excluded From the Computation of Diluted Net Loss Per Share	The following outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods presented because including them would have been antidilutive: As of December 31, 2019 2018 2018 Redeemable convertible preferred stock — 13,139,794 4,379,916 Options to purchase common stock 1,962,332 1,598,864 388,966 Restricted stock 14,625 19,492 65,510 Restricted stock acquired upon early exercise of stock options 84,964 237,853 397,993 Total 2,061,921 14,996,003 5,232,385

Supplementary Financial Data _2

Supplementary Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Supplementary Quarterly Financial Data	The following table presents the supplementary quarterly financial data for the years ended December 31, 2019 and December 31, 2018. Three Months Ended March 31 June 30 September 30 (1) December 31 2019 Total operating expenses $ 10,094 $ 11,235 $ 11,623 (1) $ 11,319 Net loss $ (9,569 ) $ (10,656 ) $ (10,969 ) $ (10,781 ) Net loss attributable to common stockholders $ (9,569 ) $ (10,656 ) $ (10,969 ) $ (10,781 ) Net loss per share attributable to common stockholders, basic and diluted $ (8.69 ) $ (1.30 ) $ (0.54 ) $ (0.53 ) 2018 Total operating expenses $ 6,087 $ 7,571 $ 13,626 $ 9,133 Net loss $ (5,737 ) $ (6,995 ) $ (13,052 ) $ (8,562 ) Net loss attributable to common stockholders $ (5,737 ) $ (6,995 ) $ (13,052 ) $ (8,562 ) Net loss per share attributable to common stockholders, basic and diluted $ (6.91 ) $ (7.47 ) $ (13.06 ) $ (8.11 ) (1) Results for the quarter ended September 30, 2018 included a $6.3 million expense related to the Novartis license fee (as further described in Note 13 to the financial statements).

Organization - Additional Infor

Organization - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	May 28, 2019	Dec. 31, 2019	Dec. 31, 2018
Subsidiary Sale Of Stock [Line Items]
Proceeds from issuance of common stock in initial public offering, net of issuance costs		$ 50,321
Accumulated deficit		92,491	$ 50,516
Cash, cash equivalents and marketable securities		$ 100,500
IPO
Subsidiary Sale Of Stock [Line Items]
Common stock, shares issued	5,750,000
Common stock, price per share	$ 10
Conversion of redeemable convertible preferred stock into common stock, shares	13,139,794
Proceeds from issuance of common stock in initial public offering, net of issuance costs	$ 50,200
Underwriting discounts and commissions	4,000
Offering costs	$ 3,300
Over Allotment Option
Subsidiary Sale Of Stock [Line Items]
Common stock, shares issued	750,000

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Details) $ in Thousands	May 21, 2019	Dec. 31, 2019USD ($)Segment	Dec. 31, 2018USD ($)	Jan. 01, 2019USD ($)
Summary Of Significant Accounting Policies [Line Items]
Reverse stock split ratio	0.0975
Number of operating segments | Segment		1
Number of reportable segments | Segment		1
Impairment of long-lived assets		$ 0	$ 0
Right-of-use assets		5,057		$ 6,100
Operating lease liabilities		$ 6,772		8,000
Deferred rent			1,900	$ 1,900
2019 Equity Incentive Plan
Summary Of Significant Accounting Policies [Line Items]
Term of options		10 years
Vesting period of options		4 years
2019 Equity Incentive Plan | Cliff Vesting
Summary Of Significant Accounting Policies [Line Items]
Vesting period of options		1 year
Minimum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of the assets		3 years
Recognized income tax positions measured at percentage of likelihood of realization		50.00%
Maximum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of the assets		5 years
IPO
Summary Of Significant Accounting Policies [Line Items]
Deferred offering costs		$ 2,700	$ 600

Fair Value Measurement - Schedu

Fair Value Measurement - Schedule of Financial Assets and Liabilities Measured and Recognized at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, amortized cost	$ 66,350	$ 69,486
Marketable securities, gross unrealized gains	69
Marketable securities, gross unrealized losses	(4)	(30)
Marketable securities, estimated fair value	66,415	69,456
Amortized Cost	100,358	89,905
Estimated Fair Value	100,423	89,875
U.S. Government Securities | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, amortized cost	24,973	9,983
Marketable securities, gross unrealized gains	32
Marketable securities, gross unrealized losses		(1)
Marketable securities, estimated fair value	25,005	9,982
Corporate Bonds | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, amortized cost	39,185	18,820
Marketable securities, gross unrealized gains	37
Marketable securities, gross unrealized losses	(4)	(19)
Marketable securities, estimated fair value	39,218	18,801
Commercial Paper | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, amortized cost	2,192
Marketable securities, estimated fair value	2,192
Commercial Paper | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, amortized cost		29,699
Marketable securities, estimated fair value		29,699
Asset-Backed Securities | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, amortized cost		10,984
Marketable securities, gross unrealized losses		(10)
Marketable securities, estimated fair value		10,974
Money Market Funds | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market funds, amortized Cost	34,008	20,419
Money market funds, estimated Fair Value	$ 34,008	$ 20,419

Fair Value Measurement - Additi

Fair Value Measurement - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial liabilities recognized at fair value	$ 0	$ 0
Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities remaining maturity period	20 months	12 months

Fair value Measurement - Sche_2

Fair value Measurement - Schedule of Changes in Fair value of Financial Liabilities (Details) - Level 3 - Redeemable Convertible Preferred Stock $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,beginning balance	$ 3,207
Issuance of redeemable convertible preferred stock	(3,137)
Change in fair value included in other income (expense), net	$ (70)

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Total property and equipment	$ 7,158	$ 6,435
Less: Accumulated depreciation and amortization	(2,516)	(1,283)
Property and equipment, net	4,642	5,152
Laboratory Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	$ 4,034	3,335
Property and equipment net, useful life	5 years
Computer Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	$ 117	117
Property and equipment net, useful life	3 years
Software
Property Plant And Equipment [Line Items]
Total property and equipment	$ 118	109
Property and equipment net, useful life	3 years
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	$ 2,581	2,570
Property and equipment net, useful life	Shorter of useful life or lease term
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	$ 308	$ 304
Property and equipment net, useful life	5 years

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Balance Sheet Related Disclosures [Abstract]
Depreciation and amortization expense	$ 1.2	$ 0.9	$ 0.4

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Balance Sheet Related Disclosures [Abstract]
Accrued salaries and benefits	$ 1,733	$ 1,279
Accrued research and development expenses	2,787	1,095
Legal and professional fees	457	999
Other	46	210
Accrued liabilities	$ 5,023	$ 3,583

Leases, Lease Commitments and_3

Leases, Lease Commitments and Contingencies - Additional Information (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Jun. 01, 2018
Operating Leased Assets [Line Items]
Penalty incurred as a result of the termination	$ 0
Rent expense			$ 1,400,000	$ 800,000
Deferred rent			$ 1,900,000		$ 1,900,000
Operating lease cost		$ 1,500,000
Right-of-use assets		5,057,000			6,100,000
Operating lease liabilities		6,772,000			$ 8,000,000
Cash paid included in operating lease liabilities included in cash flow from operating activities		$ 1,700,000
Lease Agreement | Laboratory and Office Facilities
Operating Leased Assets [Line Items]
Tenant improvement allowance				$ 1,600,000
Tenant improvement allowances amortization through date				Dec. 31, 2018
Amendment of Lease Agreement | Laboratory and Office Facilities
Operating Leased Assets [Line Items]
Tenant improvement allowance						$ 400,000
South San Francisco, California
Operating Leased Assets [Line Items]
Operating lease, remaining lease term		4 years 7 months 6 days
Operating lease, discount rate		7.00%
Operating lease option to extend		true
Operating lease renewal term		2 years
South San Francisco, California | Lease Agreement | Laboratory and Office Facilities
Operating Leased Assets [Line Items]
Lease expiration date		Jul. 31, 2024
San Diego, California | Lease Agreement | Laboratory and Office Facilities
Operating Leased Assets [Line Items]
Lease expiration date		Mar. 31, 2020

Leases, Lease Commitments and_4

Leases, Lease Commitments and Contingencies - Schedule of Future Minimum Lease Payments Under Non-cancelable Operating Leases (Details) $ in Thousands	Dec. 31, 2018USD ($)
Leases [Abstract]
2019	$ 1,711
2020	1,611
2021	1,604
2022	1,646
2023	1,690
Thereafter	1,441
Total lease payments	$ 9,703

Leases, Lease Commitments and_5

Leases, Lease Commitments and Contingencies - Schedule of Maturities of Operating Lease Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019
Leases [Abstract]
2020	$ 1,564
2021	1,604
2022	1,646
2023	1,690
2024	1,441
Total lease payments	7,945
Less: Interest	(1,173)
Operating lease liabilities	6,772	$ 8,000
Current lease liabilities	1,145
Long-term lease liabilities	5,627
Total lease liabilities	$ 6,772	$ 8,000

Income Taxes - Schedule of Prov

Income Taxes - Schedule of Provision for Income Taxes (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	21.00%	21.00%	34.00%
State income taxes	7.00%	6.90%	6.90%
Change in valuation allowance	(31.40%)	(29.40%)	(29.20%)
Research tax credits	3.60%	2.70%	2.90%
Other permanent differences	(0.40%)	(1.30%)	(2.10%)
Change in fair value of redeemable convertible preferred stock liability	0.20%	0.10%	7.00%
Remeasurement of deferred tax due to tax law change			(19.50%)
Provision for income taxes	0.00%	0.00%	0.00%

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Line Items]
Federal statutory income tax rate	21.00%	21.00%	34.00%
Tax cut and jobs act net operating loss deduction limitation percentage		80.00%
Reduction in net deferred tax assets from the prior year due to tax act			$ 2,300
Deferred tax assets, net operating loss carryforwards	$ 23,947	$ 12,633
Research and development credit carryforwards	2,995	1,458
Unrecognized tax benefits	746	$ 399	$ 174
Federal Income Tax
Income Tax Disclosure [Line Items]
Deferred tax assets, net operating loss carryforwards	85,700
Deferred tax assets, operating loss carryforwards subject to expire	$ 18,900
Tax credit carry forward expiration year	2036
Deferred tax assets, operating loss carryforwards subject to do not expire	$ 66,800
Research and development credit carryforwards	1,900
California State Income Tax
Income Tax Disclosure [Line Items]
Deferred tax assets, net operating loss carryforwards	84,600
Deferred tax assets, operating loss carryforwards subject to expire	$ 84,600
Tax credit carry forward expiration year	2036
Research and development credit carryforwards	$ 2,200

Income Taxes - Schedule of Tax

Income Taxes - Schedule of Tax Effects of Temporary Differences and Carryforwards of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforwards	$ 23,947	$ 12,633
Research and development credit carryforwards	2,995	1,458
Lease liability	1,895
Intangible assets	1,809	1,913
Accruals and reserves	436	1,048
Stock-based compensation	407
Gross deferred tax assets	31,489	17,052
Less: Valuation allowance	(29,462)	(16,289)
Deferred tax assets, net of valuation allowance	2,027	763
Deferred tax liabilities:
Right-of-use assets	(1,415)
Property and equipment	$ (612)	(724)
Stock-based compensation		$ (39)

Income Taxes - Summary of Activ

Income Taxes - Summary of Activity Related to Company's Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Balance	$ 399	$ 174
Increase related to prior year tax positions	59	12
Increase related to current year tax positions	288	213
Balance	$ 746	$ 399

Redeemable Convertible Prefer_5

Redeemable Convertible Preferred Stock - Additional Information (Details) - USD ($)	May 28, 2019	Dec. 31, 2018	Sep. 30, 2018	Mar. 31, 2018	Jan. 31, 2018	Feb. 28, 2017	Mar. 31, 2016	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Temporary Equity [Line Items]
Gross proceeds from issuance of redeemable convertible preferred stock									$ 105,358,000	$ 17,264,000
Dividend declared		$ 0
Preferred stock, conversion basis								one-for-one basis
Minimum
Temporary Equity [Line Items]
Percentage of voting power								50.00%
Proceeds of common stock public offering that triggers conversion of redeemable convertible preferred stock								$ 50,000,000
Common Stock
Temporary Equity [Line Items]
Conversion of redeemable convertible preferred stock into common stock, shares	13,139,794							13,139,794
Conversion of stock, shares converted		1							1
Percentage of outstanding shares converted to common stock								65.00%
Convertible Promissory Notes
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares converted							626,787
Series A Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares issued					1,414,811	2,122,219	1,630,910		1,414,811	2,122,219
Redeemable convertible preferred stock, shares issued price per share		$ 8.1415			$ 8.1415	$ 8.1415	$ 8.1415		$ 8.1415
Gross proceeds from issuance of redeemable convertible preferred stock					$ 11,500,000	$ 17,300,000	$ 13,300,000
Redeemable convertible preferred stock, value issued									$ 14,651,000	$ 17,399,000
Redeemable convertible preferred stock, shares authorized		59,433,105						0	59,433,105
Redeemable convertible preferred stock, par value		$ 0.0001						$ 0.0001	$ 0.0001
Redeemable convertible preferred stock, shares outstanding		5,794,727						0	5,794,727
Preferred stock, dividends per share								$ 0.6513
Series A Redeemable Convertible Preferred Stock | Convertible Promissory Notes
Temporary Equity [Line Items]
Conversion of promissory notes							$ 4,100,000
Conversion of promissory notes price per share							$ 6.5132
Series B Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares issued				753,187	6,328,265				7,081,452
Redeemable convertible preferred stock, shares issued price per share		$ 13.2769		$ 13.2769	$ 13.2769				$ 13.2769
Gross proceeds from issuance of redeemable convertible preferred stock				$ 10,000,000	$ 84,000,000
Redeemable convertible preferred stock, value issued									$ 93,844,000
Redeemable convertible preferred stock, shares authorized		75,500,000						0	75,500,000
Redeemable convertible preferred stock, par value		$ 0.0001						$ 0.0001	$ 0.0001
Redeemable convertible preferred stock, shares outstanding		7,345,067						0	7,345,067
Preferred stock, dividends per share								$ 1.0626
Series B Redeemable Convertible Preferred Stock | Novartis | License Agreement
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares issued			263,615
Redeemable convertible preferred stock, value issued			$ 3,800,000
Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized		134,933,105						0	134,933,105
Redeemable convertible preferred stock, par value		$ 0.0001						$ 0.0001	$ 0.0001
Redeemable convertible preferred stock, shares outstanding		13,139,794						0	13,139,794	4,379,916	2,257,697

Redeemable Convertible Prefer_6

Redeemable Convertible Preferred Stock - Schedule of Issued and Outstanding Redeemable Convertible Preferred Stock and its Principal Terms (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Jan. 31, 2018	Dec. 31, 2017	Feb. 28, 2017	Dec. 31, 2016	Mar. 31, 2016
Series A Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Stock Authorized	0	59,433,105
Redeemable Convertible Preferred Stock Outstanding	0	5,794,727
Liquidation Value	$ 0	$ 47,178
Carrying Value		$ 40,735
Original Issue Price Per Share		$ 8.1415		$ 8.1415		$ 8.1415		$ 8.1415
Series B Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Stock Authorized	0	75,500,000
Redeemable Convertible Preferred Stock Outstanding	0	7,345,067
Liquidation Value	$ 0	$ 97,520
Carrying Value		$ 97,656
Original Issue Price Per Share		$ 13.2769	$ 13.2769	$ 13.2769
Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Stock Authorized	0	134,933,105
Redeemable Convertible Preferred Stock Outstanding	0	13,139,794			4,379,916		2,257,697
Liquidation Value		$ 144,698
Carrying Value		$ 138,391			$ 26,084		$ 8,685

Redeemable Convertible Prefer_7

Redeemable Convertible Preferred Stock Liability - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Jan. 22, 2018	Dec. 31, 2017	Feb. 15, 2017	Jan. 31, 2018	Feb. 28, 2017	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Temporary Equity [Line Items]
Proceeds from issuance of redeemable convertible preferred stock							$ 105,358	$ 17,264
Series A Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Issuance of redeemable convertible preferred stock, value							$ 14,651	$ 17,399
Issuance of redeemable convertible preferred stock, shares				1,414,811	2,122,219	1,630,910	1,414,811	2,122,219
Redeemable convertible preferred stock issue price per share				$ 8.1415	$ 8.1415	$ 8.1415	$ 8.1415
Proceeds from issuance of redeemable convertible preferred stock				$ 11,500	$ 17,300	$ 13,300
Series A Redeemable Convertible Preferred Stock | First Tranche
Temporary Equity [Line Items]
Issuance of redeemable convertible preferred stock, value						$ 17,400
Issuance of redeemable convertible preferred stock, shares						2,257,697
Redeemable convertible preferred stock issued for conversion of promissory notes						626,787
Convertible promissory notes outstanding						$ 4,100
Redeemable convertible preferred stock issue price per share						$ 6.5132
Series A Redeemable Convertible Preferred Stock | Second Tranche Option
Temporary Equity [Line Items]
Issuance of redeemable convertible preferred stock, value						$ 17,300
Issuance of redeemable convertible preferred stock, shares			2,122,219			2,122,219
Redeemable convertible preferred stock issue price per share			$ 8.1415
Redeemable convertible preferred stock liability						$ 4,800
Remeasurement of redeemable convertible preferred stock liability									$ 2,200
Fair value reclassified to redeemable convertible preferred stock upon extinguishment			$ 100
Series A Redeemable Convertible Preferred Stock | Second Tranche Option | Other Income (Expense), Net
Temporary Equity [Line Items]
Gain on remeasurement of redeemable convertible preferred stock liability			$ 2,100
Series A Redeemable Convertible Preferred Stock | Third Tranche Option
Temporary Equity [Line Items]
Issuance of redeemable convertible preferred stock, value						$ 11,500
Issuance of redeemable convertible preferred stock, shares	1,414,811					1,414,811
Redeemable convertible preferred stock issue price per share	$ 8.1415
Redeemable convertible preferred stock liability		$ 3,200				$ 4,800		$ 3,200
Remeasurement of redeemable convertible preferred stock liability									$ 3,500
Fair value reclassified to redeemable convertible preferred stock upon extinguishment	$ 3,100
Proceeds from issuance of redeemable convertible preferred stock	11,500
Series A Redeemable Convertible Preferred Stock | Third Tranche Option | Other Income (Expense), Net
Temporary Equity [Line Items]
Gain on remeasurement of redeemable convertible preferred stock liability	$ 100	$ 300

Redeemable Convertible Prefer_8

Redeemable Convertible Preferred Stock Liability - Schedule of Redeemable Convertible Preferred Stock Liability Valuation Assumptions under OPM (Details) - $ / shares	Jan. 22, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Dividend yield		0.00%	0.00%	0.00%
Series A Redeemable Convertible Preferred Stock | Third Tranche Option
Temporary Equity [Line Items]
Stock price	$ 1.01
Expected volatility	0.00%
Risk-free interest rate	0.00%
Dividend yield	0.00%

Common Stock - Additional Infor

Common Stock - Additional Information (Details)	12 Months Ended
	Dec. 31, 2019USD ($)Vote$ / sharesshares	Dec. 31, 2018$ / sharesshares
Stockholders Equity Note [Abstract]
Common stock, shares authorized | shares	300,000,000	170,800,000
Common stock, par value | $ / shares	$ 0.0001	$ 0.0001
Common stock, voting rights	Each share of common stock is entitled to one vote.
Number of common stock voting rights held per share | Vote	1
Common stock, dividends declared | $	$ 0

Common Stock - Schedule of Numb

Common Stock - Schedule of Number of Common Stock Reserved for Future Issuance (Details) - shares	Dec. 31, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Number of common stock reserved for future issuance	3,194,078	15,274,349
Conversion of Series A Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Number of common stock reserved for future issuance		5,794,727
Conversion of Series B Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Number of common stock reserved for future issuance		7,345,067
Exercise of Outstanding Options | 2015 and 2019 Plans
Class Of Stock [Line Items]
Number of common stock reserved for future issuance	1,962,332	1,598,864
Issuance of Common Stock Options | 2015 Plan
Class Of Stock [Line Items]
Number of common stock reserved for future issuance		535,691
Issuance of Common Stock Options | 2019 Plan
Class Of Stock [Line Items]
Number of common stock reserved for future issuance	1,036,746
Issuance of Common Stock Options | Employee Stock Purchase Plan
Class Of Stock [Line Items]
Number of common stock reserved for future issuance	195,000

Stock Option Plan - Additional

Stock Option Plan - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares issued	20,339,461	1,335,690
Common stock, initially reserved for issuance	3,194,078	15,274,349
Stock-based compensation expense	$ 2,168,000	$ 950,000	$ 136,000
Weighted-average grant-date fair value of options granted	$ 6.73	$ 3.28
Aggregate intrinsic value of options exercised	$ 800,000	$ 100,000
Total unrecognized stock-based compensation expense for stock options	$ 5,900,000
Total unrecognized stock-based compensation expense, weighted-average period of recognition	2 years 9 months 21 days
Total fair value of options vested	$ 2,000,000	$ 300,000
Common stock repurchased	23,698	17,900
Stock options granted to purchase shares	667,067	1,417,674
Estimated dividend yield	0.00%	0.00%	0.00%
Restricted shares issued under founder stock purchase agreements				346,579	346,579
Issuance of Common Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares subject to repurchase	84,964	237,853
Other Current Liabilities and Other Non-Current Liabilities
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate exercise prices of early exercised shares	$ 100,000	$ 200,000
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted to purchase shares	0	7,800
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 0
Total fair value of stock vested	$ 100,000
Shares of restricted stock	14,625	19,492
Restricted Stock | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total fair value of stock vested		$ 100,000
Aggregate purchase price of unvested restricted stock subject to repurchase	$ 100,000	$ 100,000
2019 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Term of options	10 years
Vesting period of options	4 years
2019 Equity Incentive Plan | Cliff Vesting
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period of options	1 year
2019 Equity Incentive Plan | 10% Stockholder
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Term of options	5 years
2019 Equity Incentive Plan | Incentive Stock Options and Nonqualified Stock Options | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of exercise price of stock option on estimated fair value of shares	100.00%
2019 Equity Incentive Plan | Incentive Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of exercise price of stock option granted to employee	10.00%
2019 Equity Incentive Plan | Incentive Stock Options | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of exercise price of stock option on estimated fair value of shares	110.00%
2019 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, initially reserved for issuance	195,000
Offering period under ESPP	6 months
Maximum offering period under ESPP	27 months
Minimum eligible rate of compensation	1.00%
Maximum eligible rate of compensation	15.00%
2019 Employee Stock Purchase Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Ownership interest periodic payroll deduction of fair marker value	85.00%
Common stock, shares issued	4,000
Stock-based compensation expense	$ 100,000

Stock Option Plan - Summary of

Stock Option Plan - Summary of Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 2,168	$ 950	$ 136
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	956	410	80
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 1,212	$ 540	$ 56

Stock Option Plan - Summary o_2

Stock Option Plan - Summary of Activity under Stock Option Plan (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding Options, Shares, Options granted	667,067	1,417,674
Stock Option Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding Options, Shares available for Grant, Beginning balance	535,691
Outstanding Options, Shares available for Grant, Additional shares authorized	978,500
Outstanding Options, Shares available for Grant, Options granted	(667,067)
Outstanding Options, Shares available for Grant, Options repurchased	23,698
Outstanding Options, Shares available for Grant, Options canceled	165,924
Outstanding Options, Shares available for Grant, Ending balance	1,036,746	535,691
Outstanding Options, Shares, Beginning balance	1,598,864
Outstanding Options, Shares, Options granted	667,067
Outstanding Options, Shares, Options exercised	(137,675)
Outstanding Options, Shares, Options canceled	(165,924)
Outstanding Options, Shares, Ending balance	1,962,332	1,598,864
Outstanding Options, Shares, Exercisable	580,036
Outstanding Options, Shares, Vested and expected to vest	1,962,332
Outstanding Options, Weighted-Average Exercise Price, Beginning balance	$ 4.13
Outstanding Options, Weighted-Average Exercise Price, Options granted	9.66
Outstanding Options, Weighted-Average Exercise Price, Options exercised	2.24
Outstanding Options, Weighted-Average Exercise Price, Options repurchased	0.86
Outstanding Options, Weighted-Average Exercise Price, Options canceled	5.41
Outstanding Options, Weighted-Average Exercise Price, Ending balance	6.03	$ 4.13
Outstanding Options, Weighted-Average Exercise Price, Exercisable	4.28
Outstanding Options, Weighted-Average Exercise Price, Vested and expected to vest	$ 6.03
Outstanding Options, Weighted Average Remaining Contractual Term (Years)	8 years 7 months 17 days	9 years 2 months 19 days
Outstanding Options, Weighted Average Remaining Contractual Term (Years), Exercisable	8 years 2 months 15 days
Outstanding Options, Weighted Average Remaining Contractual Term (Years), Vested and expected to vest	8 years 7 months 17 days

Stock Option Plan - Assumptions

Stock Option Plan - Assumptions Used to Calculate Fair Values of Options (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	77.20%	80.30%	79.40%
Expected volatility, maximum	85.50%	82.30%	92.80%
Risk-free interest rate, minimum	1.40%	2.60%	1.90%
Risk-free interest rate, maximum	2.50%	3.10%	2.30%
Dividend yield	0.00%	0.00%	0.00%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	5 years 4 months 24 days	5 years 9 months 18 days	5 years 10 months 24 days
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days

Stock Option Plan - Summary o_3

Stock Option Plan - Summary of Restricted Stock Activity (Details) - Restricted Stock	12 Months Ended
	Dec. 31, 2019$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares of Restricted Stock Outstanding, December 31, 2018 | shares	19,492
Number of Shares of Restricted Stock Outstanding, Vested | shares	(4,867)
Number of Shares of Restricted Stock Outstanding, December 31, 2019 | shares	14,625
Weighted Average Grant Date Fair Value, Unvested, December 31, 2018 | $ / shares	$ 0.63
Weighted Average Grant Date Fair Value, Vested | $ / shares	0.05
Weighted Average Grant Date Fair Value, Unvested, December 31, 2019 | $ / shares	$ 0.82

Net Loss Per Share Attributab_3

Net Loss Per Share Attributable to Common Stockholders - Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	[1]	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss	$ (10,781)	$ (10,969)		$ (10,656)	$ (9,569)	$ (8,562)	$ (13,052)	$ (6,995)	$ (5,737)	$ (41,975)	$ (34,346)	$ (11,862)
Denominator:
Weighted-average shares outstanding										12,669,367	1,338,776	1,181,179
Less: weighted-average shares of restricted stock that are subject to repurchase										(172,410)	(382,524)	(485,807)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted										12,496,957	956,252	695,372
Net loss per share attributable to common stockholders, basic and diluted	$ (0.53)	$ (0.54)		$ (1.30)	$ (8.69)	$ (8.11)	$ (13.06)	$ (7.47)	$ (6.91)	$ (3.36)	$ (35.92)	$ (17.06)
[1]	Results for the quarter ended September 30, 2018 included a $6.3 million expense related to the Novartis license fee (as further described in Note 13 to the financial statements).

Net Loss Per Share Attributab_4

Net Loss Per Share Attributable to Common Stockholders - Schedule of Outstanding Shares of Potentially Dilutive Securities Excluded From the Computation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities	2,061,921	14,996,003	5,232,385
Issuance of Common Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities	1,962,332	1,598,864	388,966
Restricted Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities	14,625	19,492	65,510
Restricted Stock Acquired upon Early Exercise of Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities	84,964	237,853	397,993
Redeemable Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities		13,139,794	4,379,916

Net Loss Per Share Attributab_5

Net Loss Per Share Attributable to Common Stockholders - Additional Information (Details) - Employee Stock Purchase Plan $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)shares
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Contributions from plan participant in shares | shares	4,378
Stock price	85.00%
Maximum
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Contributions from plan participant | $	$ 0.1

401(k) Retirement Savings Plan

401(k) Retirement Savings Plan - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Contribution Plan Disclosure [Line Items]
Percentage of maximum annual contribution per employee	90.00%
Matching contributions amount	$ 0.2	$ 0.1
Plan name	401(k) plan	401(k) plan
Defined Contribution Plan, Sponsor Location [Extensible List]	country:US	country:US
Defined Contribution Plan, Tax Status [Extensible List]	us-gaap:QualifiedPlanMember	us-gaap:QualifiedPlanMember
Maximum
Defined Contribution Plan Disclosure [Line Items]
Matching contributions amount			$ 0.1

License Agreements - Additional

License Agreements - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended			3 Months Ended	12 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Jan. 31, 2018	Sep. 30, 2018	Dec. 31, 2018
Series B Redeemable Convertible Preferred Stock
License Agreements [Line Items]
Redeemable convertible preferred stock, shares issued		753,187	6,328,265		7,081,452
Issuance of redeemable convertible preferred stock, value					$ 93,844
Novartis
License Agreements [Line Items]
Payments for license agreement				$ 6,300
License Agreement | Novartis
License Agreements [Line Items]
Payments for license agreement	$ 2,500
License Agreement | Novartis | Maximum
License Agreements [Line Items]
Contingent development and sales milestone payments	$ 29,000
License Agreement | Novartis | Series B Redeemable Convertible Preferred Stock
License Agreements [Line Items]
Redeemable convertible preferred stock, shares issued	263,615
Issuance of redeemable convertible preferred stock, value	$ 3,800

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) - Former Director - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Accounts payable	$ 0.2
Research and Development
Related Party Transaction [Line Items]
Expenses in relation to agreements with related parties	$ 1.3	$ 0.3

Supplementary Financial Data _3

Supplementary Financial Data (Unaudited) - Supplementary Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	[1]	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Total operating expenses	$ 11,319	$ 11,623		$ 11,235	$ 10,094	$ 9,133	$ 13,626	$ 7,571	$ 6,087	$ 44,271	$ 36,417	$ 14,438
Net loss	(10,781)	(10,969)		(10,656)	(9,569)	(8,562)	(13,052)	(6,995)	(5,737)	$ (41,975)	$ (34,346)	$ (11,862)
Net loss attributable to common stockholders	$ (10,781)	$ (10,969)		$ (10,656)	$ (9,569)	$ (8,562)	$ (13,052)	$ (6,995)	$ (5,737)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.53)	$ (0.54)		$ (1.30)	$ (8.69)	$ (8.11)	$ (13.06)	$ (7.47)	$ (6.91)	$ (3.36)	$ (35.92)	$ (17.06)
[1]	Results for the quarter ended September 30, 2018 included a $6.3 million expense related to the Novartis license fee (as further described in Note 13 to the financial statements).

Supplementary Financial Data _4

Supplementary Financial Data (Unaudited) - Supplementary Quarterly Financial Data (Details) (Parenthetical) $ in Millions	3 Months Ended
	Sep. 30, 2018USD ($)
Novartis
Quarterly Financial Information [Line Items]
Payments for license agreement	$ 6.3