Harpoon Therapeutics (HARP) 10-Q 6 Nov 19 2019 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2019	Oct. 31, 2019
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Trading Symbol	HARP
Entity Registrant Name	Harpoon Therapeutics, Inc.
Entity Central Index Key	0001708493
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		24,638,428
Entity Shell Company	false
Entity Tax Identification Number	47-3458693
Entity File Number	001-38800
Entity Address, Address Line One	131 Oyster Point Blvd
Entity Address, Address Line Two	Suite 300
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	443-7400
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common stock, par value $0.0001 per share
Security Exchange Name	NASDAQ
Entity Incorporation, State or Country Code	DE
Document Quarterly Report	true
Document Transition Report	false

Condensed Balance Sheets (unaud

Condensed Balance Sheets (unaudited) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 56,764	$ 89,493
Short-term marketable securities	64,463
Prepaid expenses and other current assets	1,764	730
Total current assets	122,991	90,223
Property and equipment, net	11,770	2,998
Operating lease right-of-use asset	7,106
Tenant improvement allowance receivable		5,784
Other assets	521	3,575
Total assets	142,388	102,580
Current liabilities
Accounts payable	3,900	4,357
Accrued liabilities	7,719	3,341
Deferred revenue, current	4,080	4,250
Operating lease liabilities, current	987
Total current liabilities	16,686	11,948
Deferred revenue, noncurrent	4,420	7,792
Operating lease liabilities, net of current portion	14,051
Other long-term liabilities		6,742
Total liabilities	35,157	26,482
Commitments and contingencies (Note 7)
Convertible preferred stock, $0.0001 par value; no shares and 82,000,000 shares authorized at September 30, 2019 and December 31, 2018, respectively; no shares and 16,618,448 shares issued and outstanding at September 30, 2019 and December 31, 2018, respectively; $0 and $130,178 aggregate liquidation preference at September 30, 2019 and December 31, 2018, respectively		129,577
Stockholders' equity (deficit)
Common stock, $0.0001 par value; 150,000,000 shares and 114,000,000 shares authorized at September 30, 2019 and December 31, 2018, respectively; 24,613,803 shares and 1,383,221 shares issued and outstanding at September 30, 2019 and December 31, 2018, respectively	3	1
Additional paid-in capital	211,038	9,111
Accumulated other comprehensive income	85
Accumulated deficit	(103,895)	(62,591)
Total stockholders' equity (deficit)	107,231	(53,479)
Total liabilities, convertible preferred stock and stockholders' equity (deficit)	$ 142,388	$ 102,580

Condensed Balance Sheets (una_2

Condensed Balance Sheets (unaudited) (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, Authorized	150,000,000	114,000,000
Common stock, issued	24,613,803	1,383,221
Common stock, outstanding	24,613,803	1,383,221
Convertible Preferred Stock
Convertible preferred stock, par value	$ 0.0001	$ 0.0001
Convertible preferred stock, authorized	0	82,000,000
Convertible preferred stock, issued	0	16,618,448
Convertible preferred stock, outstanding	0	16,618,448
Convertible preferred stock, aggregate liquidation preference, value	$ 0	$ 130,178

Condensed Statements of Operati

Condensed Statements of Operations and Comprehensive Loss (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenue
Total revenue	$ 1,417	$ 1,063	$ 3,543	$ 3,689
Operating expenses
Research and development	9,533	5,967	28,886	17,651
General and administrative	8,493	1,942	18,059	3,891
Total operating expenses	18,026	7,909	46,945	21,542
Loss from operations	(16,609)	(6,846)	(43,402)	(17,853)
Interest income	727	109	2,143	248
Other expense	(26)	(22)	(45)	(29)
Net loss	(15,908)	(6,759)	(41,304)	(17,634)
Other comprehensive loss:
Net unrealized gain (loss) on marketable securities	(25)		85
Comprehensive loss	$ (15,933)	$ (6,759)	$ (41,219)	$ (17,634)
Net loss per share, basic and diluted	$ (0.65)	$ (6.23)	$ (1.95)	$ (17.22)
Weighted-average common shares used in computing net loss per share, basic and diluted	24,457,402	1,084,477	21,202,848	1,024,105
Collaboration and License
Revenue
Total revenue	$ 1,417	$ 1,063	$ 3,543	$ 3,689

Condensed Statements of Stockho

Condensed Statements of Stockholders' Equity (Deficit) (unaudited) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Note Receivable from Stockholder	Accumulated Other Comprehensive Income	Accumulated Deficit
Beginning balance, shares at Dec. 31, 2017			948,294
Beginning balances at Dec. 31, 2017	$ (26,943)		$ 1	$ 8,309	$ (28)		$ (35,225)
Temporary equity, beginning balances, shares at Dec. 31, 2017		6,989,973
Temporary equity, beginning balances at Dec. 31, 2017		$ 39,841
Issuance of common stock for exercise of stock options	7			7
Issuance of common stock for exercise of stock options, shares			10,248
Vesting of early exercised stock options	3			3
Vesting of early exercised stock options, shares			4,575
Stock-based compensation	147			147
Vesting of Founder's shares, shares			20,980
Net loss	(4,881)						(4,881)
Ending balance, shares at Mar. 31, 2018			984,097
Ending balances at Mar. 31, 2018	(31,667)		$ 1	8,466	(28)		(40,106)
Temporary equity, ending balances, shares at Mar. 31, 2018		6,989,973
Temporary equity, ending balances at Mar. 31, 2018		$ 39,841
Beginning balance, shares at Dec. 31, 2017			948,294
Beginning balances at Dec. 31, 2017	(26,943)		$ 1	8,309	(28)		(35,225)
Temporary equity, beginning balances, shares at Dec. 31, 2017		6,989,973
Temporary equity, beginning balances at Dec. 31, 2017		$ 39,841
Net loss	(17,634)
Ending balance, shares at Sep. 30, 2018			1,173,307
Ending balances at Sep. 30, 2018	(43,981)	$ 41,833	$ 1	8,877			(52,859)
Temporary equity, ending balances, shares at Sep. 30, 2018		10,118,513
Beginning balance, shares at Dec. 31, 2017			948,294
Beginning balances at Dec. 31, 2017	(26,943)		$ 1	8,309	(28)		(35,225)
Temporary equity, beginning balances, shares at Dec. 31, 2017		6,989,973
Temporary equity, beginning balances at Dec. 31, 2017		$ 39,841
Ending balance, shares at Dec. 31, 2018			1,211,419
Ending balances at Dec. 31, 2018	(53,479)		$ 1	9,111			(62,591)
Temporary equity, ending balances, shares at Dec. 31, 2018		16,618,448
Temporary equity, ending balances at Dec. 31, 2018	129,577	$ 129,577
Beginning balance, shares at Mar. 31, 2018			984,097
Beginning balances at Mar. 31, 2018	(31,667)		$ 1	8,466	(28)		(40,106)
Temporary equity, beginning balances, shares at Mar. 31, 2018		6,989,973
Temporary equity, beginning balances at Mar. 31, 2018		$ 39,841
Issuance of common stock for exercise of stock options	6			6
Issuance of common stock for exercise of stock options, shares			10,168
Vesting of early exercised stock options	17			17
Vesting of early exercised stock options, shares			28,809
Vesting of Founder's shares	140			140
Vesting of Founder's shares, shares			20,201
Net loss	(5,994)						(5,994)
Ending balance, shares at Jun. 30, 2018			1,043,275
Ending balances at Jun. 30, 2018	(37,498)		$ 1	8,629	(28)		(46,100)
Temporary equity, ending balances, shares at Jun. 30, 2018		6,989,973
Temporary equity, ending balances at Jun. 30, 2018		$ 39,841
Issuance of convertible preferred stock upon extinguishment of notes		$ 1,992
Issuance of convertible preferred stock upon extinguishment of notes, shares		3,128,540
Issuance of common stock for exercise of stock options	123			95	$ 28
Issuance of common stock for exercise of stock options, shares			95,191
Vesting of early exercised stock options	7			7
Vesting of early exercised stock options, shares			10,744
Vesting of Founder's shares	146			146
Vesting of Founder's shares, shares			24,097
Net loss	(6,759)						(6,759)
Ending balance, shares at Sep. 30, 2018			1,173,307
Ending balances at Sep. 30, 2018	(43,981)	$ 41,833	$ 1	8,877			(52,859)
Temporary equity, ending balances, shares at Sep. 30, 2018		10,118,513
Beginning balance, shares at Dec. 31, 2018			1,211,419
Beginning balances at Dec. 31, 2018	(53,479)		$ 1	9,111			(62,591)
Temporary equity, beginning balances, shares at Dec. 31, 2018		16,618,448
Temporary equity, beginning balances at Dec. 31, 2018	129,577	$ 129,577
Conversion of Series A, B, and C convertible preferred stock into common stock, shares			16,618,448
Conversion of Series A, B, and C convertible preferred stock into common stock	129,577		$ 1	129,576
Temporary equity, conversion of Series A, B, and C convertible preferred stock into common stock, shares		(16,618,448)
Temporary equity, conversion of Series A, B, and C convertible preferred stock into common stock		$ (129,577)
Issuance of common stock upon exercise of warrants, shares			563,043
Issuance of common stock upon initial public offering, net of offering costs	70,647		$ 1	70,646
Issuance of common stock upon initial public offering, net of offering costs, shares			5,769,201
Issuance of common stock for exercise of stock options	25			25
Issuance of common stock for exercise of stock options, shares			18,036
Vesting of early exercised stock options	9			9
Vesting of early exercised stock options, shares			11,900
Stock-based compensation	369			369
Vesting of Founder's shares, shares			16,302
Net loss	(13,579)						(13,579)
Other comprehensive income (loss)	26					$ 26
Ending balance, shares at Mar. 31, 2019			24,208,349
Ending balances at Mar. 31, 2019	133,595		$ 3	209,736		26	(76,170)
Beginning balance, shares at Dec. 31, 2018			1,211,419
Beginning balances at Dec. 31, 2018	(53,479)		$ 1	9,111			(62,591)
Temporary equity, beginning balances, shares at Dec. 31, 2018		16,618,448
Temporary equity, beginning balances at Dec. 31, 2018	129,577	$ 129,577
Net loss	(41,304)
Ending balance, shares at Sep. 30, 2019			24,500,027
Ending balances at Sep. 30, 2019	107,231		$ 3	211,038		85	(103,895)
Temporary equity, ending balances, shares at Sep. 30, 2019		0
Beginning balance, shares at Mar. 31, 2019			24,208,349
Beginning balances at Mar. 31, 2019	133,595		$ 3	209,736		26	(76,170)
Issuance of common stock for exercise of stock options	136			136
Issuance of common stock for exercise of stock options, shares			205,328
Vesting of early exercised stock options	9			9
Vesting of early exercised stock options, shares			11,898
Stock-based compensation	467			467
Vesting of Founder's shares, shares			5,879
Net loss	(11,817)						(11,817)
Other comprehensive income (loss)	84					84
Ending balance, shares at Jun. 30, 2019			24,431,454
Ending balances at Jun. 30, 2019	122,474		$ 3	210,348		110	(87,987)
Issuance of common stock for exercise of stock options	89			89
Issuance of common stock for exercise of stock options, shares			56,680
Vesting of early exercised stock options	9			9
Vesting of early exercised stock options, shares			11,893
Stock-based compensation	592			592
Net loss	(15,908)						(15,908)
Other comprehensive income (loss)	(25)					(25)
Ending balance, shares at Sep. 30, 2019			24,500,027
Ending balances at Sep. 30, 2019	$ 107,231		$ 3	$ 211,038		$ 85	$ (103,895)
Temporary equity, ending balances, shares at Sep. 30, 2019		0

Condensed Statements of Stock_2

Condensed Statements of Stockholders' Equity (Deficit) (unaudited) (Parenthetical) $ in Thousands	3 Months Ended
	Sep. 30, 2018USD ($)$ / shares
Series B Preferred Stock | 2016 Notes and 2017 Notes
Payments of stock issuance costs | $	$ 8
Shares issued, price per share | $ / shares	$ 6.39

Condensed Statements of Cash Fl

Condensed Statements of Cash Flows (unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities
Net loss	$ (41,304)	$ (17,635)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	1,428	461
Depreciation and amortization	400	447
Non cash lease expense	1,110
Net amortization of discounts on marketable securities	(482)
Changes in operating assets and liabilities
Prepaid expenses and other current assets	(1,033)	(852)
Other assets	3,053	(5)
Accounts payable	(697)	(10)
Accrued liabilities	4,317	92
Deferred revenue	(3,542)	(3,188)
Operating lease liabilities	141
Other long-term liabilities		8
Net cash used in operating activities	(36,609)	(20,682)
Cash flows from investing activities
Purchases of property and equipment	(3,147)	(506)
Purchases of marketable securities	(88,867)
Maturities of marketable securities	24,970
Net cash used in investing activities	(67,044)	(506)
Cash flows from financing activities
Proceeds from initial public offering of common stock, net of commissions	70,647
Proceeds from issuance of convertible preferred stock, net of issuance costs		19,991
Proceeds from issuance of common stock upon exercise of stock options, net	277	134
Payments of deferred initial public offering costs		(35)
Net cash provided by financing activities	70,924	20,090
Net decrease in cash, cash equivalents, and restricted cash	(32,729)	(1,098)
Cash, cash equivalents, and restricted cash at beginning of period	89,960	29,423
Cash, cash equivalents, and restricted cash at end of period	57,231	28,325
Supplemental disclosures of non-cash investing and financing information
Conversion of preferred stock to common stock and additional paid-in capital	129,577
Purchases of property and equipment included in accounts payable and accrued liabilities	241	118
Right-of-use asset obtained in exchange for lease obligation	8,404
Modification of operating lease	(188)
Tenant improvements provided by landlord	$ 5,723
Deferred initial public offering costs included in accounts payable and accrued liabilities		$ 465

Organization

Organization	9 Months Ended
	Sep. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	1. Description of Business Harpoon Therapeutics, Inc. (the “Company”) is a clinical-stage immunotherapy company developing a novel class of T cell engagers that harness the power of the body’s immune system to treat patients suffering from cancer and other diseases. T cell engagers are engineered proteins that direct a patient’s own T cells to kill target cells that express specific proteins, or antigens, carried by the target cells. Using a proprietary Tri-specific T cell Activating Construct (“TriTAC”), platform, the Company is developing a pipeline of novel T cell engagers, or TriTACs, initially focused on the treatment of solid tumors and hematologic malignancies. The Company was incorporated in Delaware in March 2015 and is headquartered in South San Francisco, California. Initial Public Offering On February 7, 2019, the Company’s registration statement on Form S-1 relating to its initial public offering (“IPO”) was declared effective by the Securities and Exchange Commission and shares of its common stock began trading on the Nasdaq Global Select Market (“Nasdaq”) on February 8, 2019. The public offering price of the shares sold in the IPO was $14.00 per share. The IPO closed in February 2019, pursuant to which the Company sold 5,769,201 shares of common stock, for gross proceeds of approximately $80.8 million, including the exercise in part of the underwriters’ option to purchase additional shares. The Company received net proceeds from the IPO of approximately $70.7 million, after underwriting discounts, commissions and offering costs. Immediately prior to the completion of the IPO on February 12, 2019, all outstanding shares of redeemable convertible preferred stock, including preferred stock warrants, were converted into 17,181,491 shares of common stock and $129.6 million was reclassified from temporary equity to additional paid-in capital on the balance sheet. Subsequent to the closing of the IPO, there were no shares of redeemable convertible preferred stock outstanding. Liquidity The Company has incurred significant losses and has negative cash flows from operations. As of September 30, 2019, the Company had an accumulated deficit of $103.9 million. Management expects to continue to incur additional substantial losses in the foreseeable future as a result of the Company’s research and development activities. As of September 30, 2019, the Company had cash, cash equivalents, and marketable securities of $121.2 million, which is available to fund future operations. The Company believes that the proceeds from the IPO, along with the Company’s cash and cash equivalents balance as of September 30, 2019, provide sufficient capital resources to continue its operations for at least 12 months from the issuance date of the accompanying condensed financial statements. The Company will need to raise additional capital to support the completion of its research and development activities. The Company’s activities are subject to significant risks and uncertainties, including failing to secure additional funding to continue to operationalize the Company’s current technology and to advance the development of its product candidates.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Basis of Presentation The accompanying unaudited condensed financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) for interim financial information and the instructions to Form 10-Q and Article 10 of SEC Regulation S-X for interim financial information. The accompanying unaudited condensed financial statements and notes should be read in conjunction with the audited consolidated financial statements and notes thereto contained in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018, as filed with the SEC on March 14, 2019 (the “2018 Annual Report on Form 10-K”). The condensed Balance Sheet as of December 31, 2018 was derived from the audited annual financial statements as of the period then ended. Certain information and footnote disclosures typically included in the Company's audited financial statements have been condensed or omitted. The accompanying unaudited condensed financial statements reflect all adjustments that, in the opinion of management, are necessary for a fair statement of the results of the interim periods presented. All such adjustments are of a normal recurring nature except for the impacts of adopting new accounting standards discussed below. The accompanying unaudited condensed financial results are not necessarily indicative of results expected for the full fiscal year or for any subsequent interim period. During the three and nine months ended September 30, 2019, there were no material changes to the Company's significant accounting and financial reporting policies from those reflected in the 2018 Annual Report on Form 10-K. For further information with regard to the Company’s Significant Accounting Policies, please refer to Note 2, “Summary of Significant Accounting Policies,” to the Company’s audited financial statements included in the 2018 Annual Report on Form 10-K which have been prepared in accordance with GAAP. Reverse Stock Split On January 28, 2019, the Company filed an amendment to the Company’s amended and restated certificate of incorporation to effect a reverse split of shares of the Company’s common stock and convertible preferred stock on a 4.9175-for-one basis (the “Reverse Stock Split”). The par value and the authorized number of shares of the convertible preferred stock and common stock were not adjusted in connection with the Reverse Stock Split. All references to common stock, convertible preferred stock, warrants to purchase common stock, options to purchase common stock, early exercised options, restricted stock, share data, per share data and related information contained in the accompanying condensed financial statements have been retrospectively adjusted to reflect the effect of the Reverse Stock Split for all periods presented. Use of Estimates The preparation of financial statements in conformity with 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 revenue and expenses during the reporting period. Significant estimates and assumptions made in the accompanying condensed financial statements include, but are not limited to, the fair value of common stock, the fair value of stock options, the research period of the collaboration agreement with AbbVie Biotechnology Ltd., operating lease liabilities, income tax uncertainties and certain accruals. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could differ from those estimates. Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the Company’s Chief Operating Decision Maker in making decisions regarding resource allocation and assessing performance. To date, the Company has viewed its operations and manages its business as one segment operating primarily in the United States. Cash, Cash Equivalents and Restricted Cash The Company considers all highly liquid investments purchased with original maturities of three months or less from the purchase date to be cash equivalents. Cash equivalents consist primarily of amounts invested in money market accounts and are stated at fair value. The Company maintained restricted cash of $0.5 million for both periods ended as of September 30, 2019 and The following table provides a reconciliation of cash and cash equivalents and restricted cash reported within the balance sheets that sum to the total of the same amounts shown in the statement of cash flows. As of September 30, 2019 December 31, 2018 September 30, 2018 (in thousands) Balance Sheets Cash and cash equivalents $ 56,764 $ 89,493 $ 27,858 Restricted cash (included in other assets) 467 467 467 Cash, cash equivalents and restricted cash in Statements of Cash Flows $ 57,231 $ 89,960 $ 28,325 Marketable Securities The Company generally invests its excess cash in money market funds and investment grade short- to intermediate-term fixed income securities. Such investments are included in cash and cash equivalents, short-term marketable securities or long-term marketable securities on the balance sheets. Marketable securities with a maturity date greater than 90 days and less than one year at each balance sheet date are classified as short-term. Marketable securities with a maturity date greater than one year at each balance sheet date are classified as long-term. All of the Company’s marketable securities are considered available-for-sale and are reported at fair value with unrealized gains and losses included as a component of stockholders’ equity (deficit). The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity, which is included in interest income, net on the statements of operations. Realized gains and losses and declines in value judged to be other-than-temporary, if any, on marketable securities are included in interest income, net on the statements of operations. The cost of securities sold is determined using specific identification. The Company periodically evaluates whether declines in the fair values of its marketable securities below their book value are other-than-temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss, as well as the Company’s ability and intent to hold the marketable security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the marketable security or it is more likely than not it will be required to sell any marketable securities before recovery of its amortized cost basis. Factors considered include quoted market prices, recent financial results and operating trends, implied values from any recent transactions or offers of investee securities, credit quality of debt instrument issuers, other publicly available information that may affect the value of the marketable security, duration and severity of the decline in value, and the Company’s strategy and intentions for holding the marketable security. Concentration of Credit Risk Cash, cash equivalents and marketable securities consist of financial instruments that potentially subject the Company to a concentration of credit risk, to the extent of the amounts recorded on the balance sheets. The Company minimizes the amount of credit exposure by investing cash that is not required for immediate operating needs in money market funds and marketable securities. Leases The Company evaluates arrangements at inception to determine if an arrangement is or contains a lease. Operating lease assets represent the Company’s right to use an underlying asset for the lease term and operating lease liabilities represent the Company’s obligation to make lease payments arising from the lease. Operating lease assets and liabilities are recognized at the commencement date of the lease based upon the present value of lease payments over the lease term. When determining the lease term, the Company includes options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. The Company uses an incremental borrowing rate that the Company would expect to incur for a fully collateralized loan over a similar term under similar economic conditions to determine the present value of the lease payments. The lease payments used to determine the Company’s operating lease assets may include lease incentives and stated rent increases and are recognized in the Company’s operating lease assets in the condensed balance sheets. Operating lease liabilities are accreted over the term of the lease using the incremental borrowing rate and the associated expense is recorded to operating expenses in the condensed statement of operations and comprehensive loss. Operating lease assets are amortized to rent expense over the lease term and included in operating expenses in the condensed statement of operations and comprehensive loss. Variable lease payments are recognized as the associated obligation is incurred. Fair Value Measurement Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability, or an exit price, in the principal or most advantageous market for that asset or liability in an orderly transaction between market participants on the measurement date, and established a fair value hierarchy that requires an entity to maximize the use of observable inputs, where available, and minimize the use of unobservable inputs when measuring fair value. The Company measures fair value based on a three-level hierarchy of inputs, of which the first two are considered observable and the last unobservable. Unobservable inputs reflect the Company’s own assumptions about current market conditions. The three-level hierarchy of inputs is as follows: Level 1—Observable inputs such as unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date; Level 2—Inputs (other than quoted prices included in Level 1) are either directly or indirectly observable for the asset or liability. These include quoted prices for similar assets or liabilities in active markets and quoted prices for identical or similar assets or liabilities in markets that are not active; and Level 3— Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The carrying amounts reflected in the accompanying balance sheets for cash and cash equivalents, short-term marketable securities, prepaid expenses, other current assets, accounts payable, accrued expenses and other current liabilities approximate their fair values due to their short-term nature. Property and Equipment, Net Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation is computed on a straight-line basis over the estimated useful lives of the related assets, generally three to five years. Leasehold improvements are amortized using the straight-line method over the shorter of the assets’ estimated useful lives or the remaining term of the lease. Depreciation and amortization begin at the time the asset is placed in service. Maintenance and repairs are charged to operations as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the balance sheet and the resulting gain or loss is reflected in operations. Impairment of Long-Lived Assets The Company evaluates its long-lived assets, including property and equipment, for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets or group of assets may not be fully recoverable. If indicators of impairment exist and the undiscounted future cash flows that the assets are expected to generate are less than the carrying amount of the assets, the Company reduces the carrying amount of the assets through an impairment charge to their estimated fair values based on a discounted cash flow approach or, when available and appropriate, to comparable market values. There were no impairments of long-lived assets for any of the periods presented. Revenue Recognition Effective January 1, 2017, the Company early adopted Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers The Company enters into corporate collaborations under which it may obtain upfront license fees, research and development funding, and development, regulatory and commercial milestone payments and royalty payments. The Company’s performance obligations under these arrangements may include licenses of intellectual property, distribution rights, research and development services, delivery of manufactured product and/or participation on joint steering committees. Licenses of intellectual property: If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenue from upfront license fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. For licenses that are bundled with other promises, the Company utilizes judgement to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring proportional performance for purposes of recognizing revenue from non-refundable, up-front fees. The Company evaluates the measure of proportional performance each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Milestone payments: At the inception of each arrangement that includes development, regulatory or commercial milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price. Topic 606 suggests two alternatives to use when estimating the amount of variable consideration: the expected value method and the most likely amount method. Under the expected value method, an entity considers the sum of probability-weighted amounts in a range of possible consideration amounts. Under the most likely amount method, an entity considers the single most likely amount in a range of possible consideration amounts. Whichever method is used, it should be consistently applied throughout the life of the contract; however, it is not necessary for the Company to use the same approach for all contracts. The Company expects to use the most likely amount method for development and regulatory milestone payments. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s or the licensee’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The transaction price is then allocated to each performance obligation on a relative stand-alone selling price basis. The Company recognizes revenue as or when the performance obligations under the contract are satisfied. At the end of each subsequent reporting period, the Company re-evaluates the probability or achievement of each such milestone and any related constraint, and if necessary, adjusts its estimates of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. Commercial milestones and royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and in which the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue when the related sales occur. To date, the Company has not recognized any royalty revenue resulting from its collaboration arrangements. Upfront payments and fees are recorded as deferred revenue upon receipt or when due, and may require deferral of revenue recognition to a future period until the Company performs its obligations under these arrangements. Amounts due to the Company are recorded as accounts receivable when the Company’s right to consideration is unconditional. Research and Development Expenses and Accrued Research and Development Costs The Company expenses research and development costs as incurred. Research and development expenses consist of personnel costs for the Company’s research and product development employees. Also included are non-personnel costs such as professional fees payable to third parties for preclinical studies, clinical trials, research services, laboratory supplies and equipment maintenance and depreciation, intellectual property licenses and other consulting costs. The Company estimates preclinical and clinical study and research expenses based on the services performed, pursuant to contracts with research institutions that conduct and manage preclinical studies, clinical trials and research services on its behalf. The Company estimates these expenses based on discussions with internal management personnel and external service providers as to the progress or stage of completion of services and the contracted fees to be paid for such services. The Company records the estimated costs of research and development activities based upon the estimated amount services provided but not yet invoiced, and includes these costs in development expenses. The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its third-party service provides under the service agreements. The Company makes significant judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, the Company adjusts its accrued liabilities. The Company has not experienced any material differences between accrued costs and actual costs incurred. However, the status and timing of actual services performed may vary from the Company’s estimates, resulting in adjustments to expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect the Company’s results of operations. Payments associated with licensing agreements to acquire exclusive license to develop, use, manufacture and commercialize products that have not reached technological feasibility and do not have alternate future use are expensed as incurred. Payments made to third parties under these arrangements in advance of the performance of the related services by the third parties are recorded as prepaid expenses until the services are rendered. Such payments are evaluated for current or long-term classification based on when such services are expected to be received. Stock-Based Compensation The Company maintains a stock-based compensation plan as a long-term incentive for employees, consultants and members of the Company’s board of directors (the “Board”). The plan allows for the issuance of non-statutory options (“NSOs”) and incentive stock options to employees and NSOs to nonemployees. Share-based payments are measured using fair-value-based measurements and recognized as compensation expense over the service period in which the awards are expected to vest. The Company’s fair-value-based measurements of awards to employees and directors as of the grant date utilize the single-option award-valuation approach, and the Company uses the straight-line method for expense attribution. The fair-value-based measurements of options granted to nonemployees are remeasured at each period end until the options vest and are amortized to expense as earned. The valuation model used for calculating the estimated fair value of stock awards is the Black-Scholes option-pricing model. The Black-Scholes model requires the Company to make assumptions and judgments about the variables used in the calculations, including the expected term (weighted-average period of time that the options granted are expected to be outstanding), the expected volatility of the Company’s common stock, the related risk-free interest rate and the expected dividend. The Company has elected to recognize forfeitures of share-based payment awards as they occur. Effective January 1, 2018, the Company early adopted ASU No. 2018-07, Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting Convertible Preferred Stock The Company records all shares of convertible preferred stock at their respective fair values less issuance costs on the dates of issuance. The convertible preferred stock is recorded outside of stockholders’ deficit because, in the event of certain deemed liquidation events considered not solely within the Company’s control, such as a merger, acquisition and sale of all or substantially all of the Company’s assets, the convertible preferred stock will become redeemable at the option of the holders. In the event of a change of control of the Company, proceeds received from the sale of such shares will be distributed in accordance with the liquidation preferences set forth in the Company’s Amended and Restated Certificate of Incorporation unless the holders of convertible preferred stock have converted their shares of convertible preferred stock into shares of common stock. The Company has determined not to adjust the carrying values of the convertible preferred stock to the liquidation preferences of such shares because of the uncertainty of whether or when such an event would occur. Income Taxes Income taxes are accounted for under the asset and liability method. Under this method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates applied to taxable income in the years in which those temporary differences are expected to be realized. The effect on deferred tax assets and liabilities of a change in tax rates is recognized as income in the period that includes the enactment date. A valuation allowance is established when necessary to reduce deferred tax assets to the amount expected to be realized. Financial statement effects of uncertain tax positions are recognized when it is more-likely-than-not, based on the technical merits of the position, that it will be sustained upon examination. Interest and penalties related to unrecognized tax benefits are included as a component of other expense. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits. The Company accounts for uncertain tax positions in accordance with ASC 740-10, Accounting for Uncertainty in Income Taxes The Company includes any penalties and interest expense related to income taxes as a component of provision for income tax as necessary. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits. Net Loss Per Share Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration for potentially dilutive securities. As discussed in Note 9, the unvested portion of early exercised stock options are excluded from the computation of weighted average shares as the continuing vesting of such shares is contingent on the holders’ continued service to the Company. Diluted net loss per share is the same as basic net loss per share for each period presented, since the effects of potentially dilutive securities are antidilutive given the net loss of the Company. Comprehensive Income (Loss) Comprehensive loss includes net loss and certain changes in stockholders’ equity (deficit) that are excluded from net loss, primarily unrealized gains or losses on the Company’s marketable securities. Deferred Offering Costs The Company had deferred offering costs consisting of legal, accounting and other fees and costs directly attributable to the IPO, which was completed in February 2019. The deferred offering costs were offset against the gross proceeds of the IPO. As of September 30, 2019 and December 31, 2018, zero and $3.0 million of deferred offering costs were included in other assets on the balance sheets, respectively. Emerging Growth Company Status The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 (the “JOBS Act”). Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act until such time as those standards apply to private companies. The Company has elected to use this extended transition period for complying with new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that the Company (i) is no longer an emerging growth company or (ii) affirmatively and irrevocably opts out of the extended transition period provided in the JOBS Act. As a result, the accompanying condensed financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates. As described in “Recently Adopted Accounting Pronouncements” below, the Company early adopted Revenue from Contracts with Customers Stock Compensation—Improvements to Employee Share-Based Payment Accounting Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting, and (Topic 842), Leases as the JOBS Act does not preclude an emerging growth company from adopting a new or revised accounting standard earlier than the time that such standard applies to private companies. The Company expects to use the extended transition period for any other new or revised accounting standards during the period in which it remains an emerging growth company. Recently Adopted Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2016-02 (Topic 842), Leases (“ASU 2016-02”) In February 2018, the FASB issued ASU No. 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income Recently Issued Accounting Pronouncements In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments ASU 2018-19 ASU 2019-04 In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. The following table presents information about the Company’s financial assets that are measured at fair value and indicates the fair value hierarchy of the valuation: Fair Value Measurements at September 30, 2019 Total Level 1 Level 2 Level 3 (in thousands) Assets Cash Equivalents: Money market funds $ 39,879 $ 39,879 $ — $ — U.S. government treasuries 14,979 14,979 — — Short-term marketable securities U.S. government treasuries 27,955 27,955 — — Corporate debt securities 19,010 — 19,010 — U.S. government agency securities 17,498 — 17,498 — Total assets $ 119,321 $ 82,813 $ 36,508 $ — Fair Value Measurements at December 31, 2018 Total Level 1 Level 2 Level 3 (in thousands) Assets Cash Equivalents: Money market funds $ 60,396 60,396 $ — $ — Total assets $ 60,396 $ 60,396 $ — $ — The Company’s Level 2 securities are valued using third-party pricing sources. The pricing services utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly. The Company had no Level 3 assets or liabilities as of September 30, 2019 and December 31, 2018. There were no transfers between Level 1 and Level 2 for the periods ended September 30, 2019 and December 31, 2018. The Company did not have any financial liabilities subject to fair value measurements on a recurring basis as of September 30, 2019 and December 31, 2018.

Available-for-Sale Securities

Available-for-Sale Securities	9 Months Ended
	Sep. 30, 2019
Investments Debt And Equity Securities [Abstract]
Available-for-Sale Securities	4. All marketable securities were considered available-for-sale at September 30, 2019. The amortized cost, gross unrealized holding gains or losses, and fair value of the Company’s marketable securities by major security type at each balance sheet date are summarized in the tables below : September 30, 2019 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value (in thousands) Cash equivalents U.S. government treasuries $ 14,977 $ 2 $ — $ 14,979 Money market funds 23,782 — — 23,782 Total cash equivalents 38,759 2 — 38,761 Short-term marketable securities: U.S. government treasuries 27,944 11 — 27,955 U.S. government agency securities 17,476 21 — 17,497 Corporate debt securities 18,960 51 — 19,011 Total short-term marketable securities 64,380 83 — 64,463 Total $ 103,139 $ 85 $ — $ 103,224 December 31, 2018 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value (in thousands) Cash equivalents Money market funds $ 60,396 $ — $ — $ 60,396 Total cash equivalents $ 60,396 $ — $ — $ 60,396 As of September 30, 2019 and December 31, 2018, there were no marketable securities that were in an unrealized loss position. All of the Company’s marketable securities have an effective maturity of less than two years.

Balance Sheet Components

Balance Sheet Components	9 Months Ended
	Sep. 30, 2019
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Components	5. Balance Sheet Components Property and Equipment, Net Property and equipment, net consists of the following: September 30, December 31, 2019 2018 (in thousands) Laboratory equipment $ 3,679 $ 2,402 Furniture and fixtures 552 312 Computer equipment and software 91 32 Leasehold improvements 8,862 360 Construction in progress — 906 13,184 4,012 Less: Accumulated depreciation and amortization (1,414 ) (1,014 ) Total property and equipment, net $ 11,770 $ 2,998 Depreciation and amortization expense for property and equipment amounted to $1.1 million and $0.2 million for the three months ended September 30, 2019 and 2018, respectively and $0.9 million and $0.4 million for the nine months ended September 30, 2019 and 2018, respectively. Accrued Liabilities Accrued liabilities consist of the following: September 30, December 31, 2019 2018 Accrued research and development $ 2,016 $ 504 Accrued personnel costs 1,695 1,593 Accrued professional and consulting fees 2,663 333 Accrued offering costs — 709 Other 1,345 202 Total accrued liabilities $ 7,719 $ 3,341

Convertible Notes

Convertible Notes	9 Months Ended
	Sep. 30, 2019
Debt Disclosure [Abstract]
Convertible Notes	6 . In May 2017, the outstanding principal balance of the Company’s convertible promissory notes issued in November 2016 (the “2016 Notes”) and January 2017 (the “2017 Notes”), and the then-outstanding balance of accrued interest, converted into 811,103 shares of the Company’s Series B convertible preferred stock in conjunction with the Series B preferred stock financing. The conversion of the 2016 Notes and the 2017 Notes into shares of Series B convertible preferred stock was accounted for as an extinguishment. Upon the closing of the IPO, all shares of convertible preferred stock then outstanding were automatically converted into shares of common stock. The Company had no convertible preferred stock outstanding as of September 30, 2019. Warrants Issued with 2016 and 2017 Notes As of December 31, 2018, warrants for the purchase of an aggregate of 565,270 shares of the Company’s common stock were outstanding and exercisable. In connection with the IPO, all of these warrants automatically net exercised at the IPO price of $14.00 per share, resulting in the issuance of 563,043 shares of common stock.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	7 . Leases In February 2017, the Company entered into an operating lease agreement with Tizona Therapeutics, Inc. (the “Tizona Lease”) for its headquarters in South San Francisco, California. One member of the Board is also the Executive Chairman of Tizona and, as such, Tizona is deemed to be a related party. The lease term was for 36 months. In June 2019, the Company entered into a sublease termination agreement with Tizona (the “Termination Agreement”) with the purpose of terminating the existing Tizona Lease. Under the Termination Agreement, the Company and Tizona agreed to terminate the obligations, liabilities and benefits under the Tizona Lease as of the reduced lease term date of July 31, 2019. In addition, the Company paid a termination fee of $0.4 million to Tizona in July of 2019. The Company is no longer a subtenant to Tizona under the Tizona Lease as of September 30, 2019. In August 2018, the Company entered into a lease agreement for the office and laboratory space in South San Francisco, California (the “Cove Lease”). The lease has an initial term of eight years, beginning on the lease commencement date, with an option to extend the lease for an additional period of eight years. The lease commencement date was July 1, 2019 at which time the Company took occupancy. Pursuant to the terms of the lease, the Company is entitled to a tenant improvement allowance of approximately $5.2 million with the option for an additional tenant improvement allowance of approximately $1.4 million. The additional tenant improvement allowance of $1.4 million, which was exercised in December 2018, is treated as a loan from the landlord and is expected to be paid back (including interest) by the Company through additional rental payments. As of September 30, 2019, the full tenant improvement allowance of $6.6 million was utilized under this lease, which was recorded as leasehold improvements and a reduction to the tenant improvement allowance receivable on the balance sheet. The Cove Lease has a remaining term of As of September 30, 2019 Assets: Operating lease right-of-use assets $ 7,106 Liabilities Operating lease liabilities $ 987 Operating lease liabilities, net of current portion 14,051 Total operating lease liabilities $ 15,038 The Company incurred $0.1 million and $0.3 million in variable lease costs for each of the three and nine months periods ended September 30, 2019, respectively. F uture minimum lease payments under the Cove Lease as of September 30, 2019 are as follows (in thousands): As of September 30, 2019 Operating Lease Commitments 2019 $ 427 2020 2,487 2021 2,566 2022 2,647 2023 2,731 Thereafter 10,251 Total future minimum lease payments 21,109 Less: Present value adjustment for minimum lease commitments (6,071 ) Total $ 15,038 As of September 30, 2019, the weighted average remaining lease term was 7.80 years and the weighted average incremental borrowing rate used to determine the operating lease liability was 8.95%. Rent expense was $0.9 million and $0.3 million for the three months ended September 30, 2019 and 2018, respectively, and $2.2 million and $0.8 million for the nine months ended September 30, 2019 and 2018, respectively. Amortization of the right-of-use lease assets was $0.3 million and zero for the three months ended September 30, 2019 and 2018, respectively and $1.1 million and zero for the nine months ended September 30, 2019 and 2018, respectively. ASC 840 Disclosures The future minimum lease payments under the Company’s operating leases as of December 31, 2018, were as follows (in thousands): As of December 31, 2018 Operating Lease Commitments 2019 $ 1,308 2020 2,762 2021 2,566 2022 2,647 2023 2,731 Thereafter 10,251 Total $ 22,265 Rent expense was $1.2 million for the year ended December 31, 2018. Indemnification In the ordinary course of business, the Company enters into agreements that may include indemnification provisions. Pursuant to such agreements, the Company may indemnify, hold harmless and defend an indemnified party for losses suffered or incurred by the indemnified party. Some of the provisions will limit losses to those arising from third-party actions. In some cases, the indemnification will continue after the termination of the agreement. The maximum potential amount of future payments the Company could be required to make under these provisions is not determinable. The Company has never incurred material costs to defend lawsuits or settle claims related to these indemnification provisions. The Company has also entered into indemnification agreements with its directors and officers that may require the Company to indemnify its directors and officers against liabilities that may arise by reason of their status or service as directors or officers to the fullest extent permitted by Delaware corporate law. The Company currently has directors’ and officers’ insurance.

Collaboration & License Agreeme

Collaboration & License Agreements	9 Months Ended
	Sep. 30, 2019
Collaboration And License Agreements [Abstract]
Collaboration & License Agreements	8 . Collaboration Agreement with AbbVie On October 10, 2017, the Company entered into a Discovery Collaboration and License Agreement (the “Collaboration Agreement”) with AbbVie Biotechnology Ltd. (“AbbVie”). Pursuant to the Collaboration Agreement, the Company granted to AbbVie worldwide exclusive rights to develop and commercialize products that incorporate its proprietary TriTAC technology together with soluble T cell receptors (“TCRs”) provided by AbbVie that bind to targets accepted by the parties. Under the terms of the Collaboration Agreement, AbbVie is allowed to designate up to two targets, subject to confirmation of target availability. During a period of up to four years following the date of AbbVie’s designation of each target for the products, and confirmation of target availability, the Company and AbbVie will conduct certain research and discovery activities under a mutually agreed discovery and research plan in connection with the creation and evaluation of constructs comprising the Company’s proprietary TriTAC technology in conjunction with the soluble TCR sequences directed against the agreed upon targets of interest. The Company may not, by itself or through any third party, develop or commercialize any competing product that binds to any of the included targets. Following the discovery phase, AbbVie will be solely responsible, at its cost, for the development, manufacture and commercialization of products that arise from the activities under the discovery plan. AbbVie is required to use commercially reasonable efforts to develop and commercialize one such product directed to each target for which the discovery activities were completed, in the United States and specified European markets. In addition to an upfront payment of $17 million, AbbVie will be required to make further payments to the Company of up to $600 million in the aggregate, for the achievement of specified development, regulatory, and commercial sale milestones for licensed products indicated for human therapeutic or prophylactic use, if such licensed products are successfully progressed against all included targets and indications. The Company will also receive tiered royalties on net sales by AbbVie, its affiliates and sublicensees of licensed products at percentages in the mid-single digits, subject to specified offsets and reductions. If licensed products are developed and commercialized for diagnostic or veterinary use, or certain screening or monitoring uses, the parties have agreed to negotiate an appropriate reduction in the economic terms applicable to such non-therapeutic and prophylactic applications. In April 2019, the Company and AbbVie amended the Collaboration Agreement to modify certain time periods for selection of collaboration targets by AbbVie. The first target was selected in 2017 and the Company believes it has satisfied all of its development obligations associated with the first target as of September 30, 2019. In October 2019, AbbVie selected a second target, which allows both the Company and AbbVie to begin research and discovery activities associated with the second target. The Collaboration Agreement will terminate upon the date of the expiration of all AbbVie’s royalty payment obligations in all countries. The Collaboration Agreement may be terminated by either party immediately for the insolvency of the other party or on 90 days’ written notice for an uncured material breach of the Collaboration Agreement by the other party. In the case of a material breach with respect to commercialization diligence with respect to any major market, or with respect to only one of the included targets, the Company may terminate the Collaboration Agreement solely with respect to the affected major markets, or target, as applicable. AbbVie may also terminate the Collaboration Agreement in its entirety or on a target-by-target or country-by-country basis for any reason on 30 days’ written notice to the Company. In addition, AbbVie may terminate the Collaboration Agreement immediately in its entirety or on a target-by-target basis if AbbVie considers in good faith that there has been a failure of the discovery or development efforts with respect to such target, or that further development or commercialization of products directed to such target is not advisable as a result of a serious safety issue. The Company assessed the Collaboration Agreement in accordance with Topic 606 and concluded that AbbVie is a customer. The Company concluded that there are multiple promises under the Collaboration Agreement which include the research and development activities, regulatory documentation and know-how, exclusivity, and the license to the related technology. The Company combined these promises into a single performance obligation, as the Company is obliged to render specialized services for the research program, and other promises have either no significant value or are not distinct. The Company estimates that the $17.0 million upfront payment will be recognized over the estimated four-year period of ongoing research and development activities based on the projected activities to be performed over each reporting period relative to the total estimated performance period. Such estimates are reviewed by the Company on a periodic basis and, if necessary, the Company will adjust the measure of performance and related revenue recognition. At the inception of the Collaboration Agreement, the Company determined that the transaction price was $17.0 million, which was all allocated to the single unit of accounting. The Company has evaluated the transaction price and has determined $17.0 million is still appropriate as of September 30, 2019. For the three months ended September 30, 2019 and 2018, $1.4 million and $1.1 million of revenue has been recognized, respectively, and for the nine months ended September 30, 2019 and 2018, $3.5 million and $3.7 million of revenue has been recognized in the accompanying statement of operations and comprehensive loss, respectively. As of September 30, 2019, the Company has recorded $8.5 million in deferred revenue, of which $4.4 million is classified as long-term and $4.1 million as short-term deferred revenue, in the accompanying balance sheet. The Company determined that future contingent payments that may be received related to development and regulatory milestones under the Collaboration Agreement are based on the performance of AbbVie and are constrained due to the fact that it was not probable that a significant reversal of cumulative revenue would not occur, as their achievement is highly dependent on the successful completion of the research activities. Accordingly, revenue for the achievement of these milestones will be recognized in the period that it is deemed probable that the milestone will be achieved. Any consideration related to commercialization and sales milestones, and sales-based royalties will be recognized when the related sales occur as they were determined to relate predominantly to the license granted to AbbVie and have been excluded from the transaction price. The Company will re-evaluate the transaction price in each reporting period as uncertain events are resolved or other changes in circumstances occur. As of September 30, 2019, the Company has not recognized or earned any milestone payments under the Collaboration Agreement. The Company will recognize royalty revenue in the period of sale of the related products, based on the underlying contract terms. No such amounts were recognized during the three and nine months ended September 30, 2019. License Agreement with Werewolf Therapeutics, Inc. In March 2018, the Company entered into an assignment and license agreement (the “Werewolf Agreement”) with Werewolf Therapeutics, Inc. (“Werewolf”). Werewolf is affiliated with a holder of more than 5% of the Company’s capital stock, and one member of the Board is also the interim Chief Executive Officer of Werewolf and as such, Werewolf was deemed to be a related party. Pursuant to the Werewolf Agreement, the Company assigned certain patents and granted to Werewolf a non-exclusive, royalty-bearing, sublicensable license under certain other patents. In addition, Werewolf assigned certain patents to the Company. Under the Werewolf Agreement, the Company received an upfront fee of $0.5 million. If Werewolf commercializes products covered by the licensed patents, then beginning on the first sale of such products, the Company will be eligible to receive a royalty on net sales of such products by Werewolf, its affiliates and licensees at a percentage in the low single digits, subject to a minimum annual royalty payment at an amount in the low hundreds of thousands of dollars. The Werewolf Agreement will continue on a country-by-country basis until the expiration of the last to expire patent licensed under the Werewolf Agreement, and it may be terminated by either party for the other party’s uncured material breach. The Company may terminate the license grant to Werewolf upon Werewolf’s insolvency. In addition, Werewolf may terminate the Werewolf Agreement for convenience at any time upon advance notice to the Company. Upon any termination, the license granted to Werewolf under the Werewolf Agreement will immediately terminate. The Company assessed the Werewolf Agreement in accordance with Topic 606 and concluded that Werewolf is a customer, and there is only one promise and a performance obligation to deliver intellectual property license. The upfront fee of $0.5 million was recognized upfront during the nine month period ended September 30, 2018 upon delivery of the license to Werewolf and royalties on net sales will be recognized when the underlying sales occur. No royalty revenue was recognized under the Werewolf Agreement during the three and nine months ended September 30, 2019 or 2018.

Equity

Equity	9 Months Ended
	Sep. 30, 2019
Equity [Abstract]
Equity	9. Stock-Based Compensation As of September 30, 2019, there were 2,113,843 shares reserved by the Company to grant under the 2019 Equity Incentive Plan (the “2019 Plan”). The following summarizes option activity under the 2019 Plan: Weighted Weighted Average Number of Average Remaining Aggregate Outstanding Exercise Contractual Intrinsic Options Price Life Value (in years) (in thousands) Balance as of December 31, 2018 3,323,988 1.61 9.07 $ 1,675 Options granted 302,412 13.20 Options exercised (280,044 ) 0.89 Options cancelled (83,864 ) 1.32 Balance as of September 30, 2019 3,262,492 2.75 8.52 $ 35,661 Vested and expected to vest as of September 30, 2019 3,262,492 2.75 8.52 $ 35,661 Exercisable as of September 30, 2019 684,569 1.61 7.63 $ 8,254 As of September 30, 2019, 2,340,438 shares were authorized for the grant of options. The aggregate intrinsic values of options outstanding, vested and exercisable, and vested and expected to vest were calculated as the difference between the exercise price of the options and the estimated fair value of the Company’s common stock. The intrinsic value of options exercised was $0.7 million and $3.1 million for the three and nine months ended September 30, 2019, respectively, and $0.1 million for each of the three and nine months ended September 30, 2018. There is no future tax benefit related to options exercised, as the Company had accumulated net operating losses at September 30, 2019 and 2018. During the three months ended September 30, 2019 and 2018, the estimated weighted-average grant-date fair value of the employee options vested was $0.86 and $0.71 per share, respectively, and the estimated weighted-average grant-date fair value of the employee options granted was $9.80 and $1.20 per share, respectively. During the nine months ended September 30, 2019 and 2018, the estimated weighted-average grant-date fair value of employee options vested was $0.86 and $0.71, respectively, and the estimated weighted-average grant-date fair value of the employee options granted was $9.29 and $1.18 per share, respectively. As of September 30, 2019, there was $4.4 million of unrecognized stock-based compensation related to unvested stock options that is expected to be recognized over a weighted-average period of 2.7 years. The fair value of employee and director stock option awards was estimated at the date of grant using a Black-Scholes option-pricing model with the following weighted-average assumptions: Nine Months Ended September 30, 2019 2018 Expected term (years) 5.86 6.08 Expected volatility 77.76 % 75.03 % Risk-free interest rate 2.22 % 2.80 % Expected dividend 0 % 0 % Total stock-based compensation was as follows: Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 (in thousands) Operating Expenses Research and development $ 169 $ 71 $ 466 $ 211 General and administrative 423 103 962 250 Total stock-based compensation $ 592 $ 174 $ 1,428 $ 461 Stock-based compensation related to non-employee awards, which is included in the table above, was zero and $0.1 million for the three and nine months ended September 30, 2019 and 2018, respectively. Restricted Stock In 2015, the Company issued restricted stock awards to employees and directors under the Company’s 2015 Equity Incentive Plan (the “2015 Plan”) at a purchase price of $0.0005 per share. The shares related to restricted stock awards are subject to a lapsing repurchase right upon termination of employment at the original purchase price. In order to vest, the holders are required to provide continued service to the Company. For accounting purposes, unvested restricted stock awards are not considered issued and outstanding and therefore are not reflected as issued and outstanding in the accompanying statements of convertible preferred stock and stockholders’ equity (deficit) until the awards vest. A summary of restricted stock activity is shown in the below table: Number of of Restricted Stock Outstanding Restricted shares- December 31, 2018 22,178 Restricted stock awards vested (22,178 ) Unvested shares repurchased — Restricted shares- September 30, 2019 — Early Exercised Stock Options The terms of the 2015 Plan permit option holders to exercise stock options before they are vested, subject to certain limitations. The shares related to early exercised stock options are subject to the Company’s lapsing repurchase right upon termination of employment at the original purchase price. In order to vest, the holders are required to provide continued service to the Company. The proceeds are initially recorded in other current liabilities and are reclassified to common stock and paid-in capital as the repurchase right lapses. As of September 30, 2019 and December 31, 2018, there was $161,000 and $188,000, respectively, recorded in other current liabilities relating to shares subject to repurchase. For accounting purposes, unvested early exercised shares are not considered issued and outstanding until the awards vest. As a result of early exercises under the 2015 Plan, 113,856 and 149,565 shares had not vested and were subject to repurchase as of September 30, 2019 and December 31, 2018, respectively.

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
Net Loss Per Share	1 0. The following outstanding potentially dilutive common stock equivalents have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect: As of September 30, As of December 31, 2019 2018 Convertible preferred stock (as converted) — 16,618,448 Common stock options issued and outstanding 3,262,492 3,323,988 Restricted Stock subject to future vesting — 22,178 Early exercised stock options subject to future vesting 113,856 149,565 Warrants to purchase shares of common stock — 565,270 Total 3,376,348 20,679,449 Neither the Company’s convertible preferred stock nor restricted stock subject to future vesting participates in losses. The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company (in thousands, except per share data): Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Net loss $ (15,908 ) $ (6,759 ) $ (41,304 ) $ (17,634 ) Net loss per share, basic and diluted $ (0.65 ) $ (6.23 ) $ (1.95 ) $ (17.22 ) Weighted-average shares used to compute basic and diluted net loss per share 24,457,402 1,084,477 21,202,848 1,024,105

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) for interim financial information and the instructions to Form 10-Q and Article 10 of SEC Regulation S-X for interim financial information. The accompanying unaudited condensed financial statements and notes should be read in conjunction with the audited consolidated financial statements and notes thereto contained in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018, as filed with the SEC on March 14, 2019 (the “2018 Annual Report on Form 10-K”). The condensed Balance Sheet as of December 31, 2018 was derived from the audited annual financial statements as of the period then ended. Certain information and footnote disclosures typically included in the Company's audited financial statements have been condensed or omitted. The accompanying unaudited condensed financial statements reflect all adjustments that, in the opinion of management, are necessary for a fair statement of the results of the interim periods presented. All such adjustments are of a normal recurring nature except for the impacts of adopting new accounting standards discussed below. The accompanying unaudited condensed financial results are not necessarily indicative of results expected for the full fiscal year or for any subsequent interim period. During the three and nine months ended September 30, 2019, there were no material changes to the Company's significant accounting and financial reporting policies from those reflected in the 2018 Annual Report on Form 10-K. For further information with regard to the Company’s Significant Accounting Policies, please refer to Note 2, “Summary of Significant Accounting Policies,” to the Company’s audited financial statements included in the 2018 Annual Report on Form 10-K which have been prepared in accordance with GAAP.
Reverse Stock Split	Reverse Stock Split On January 28, 2019, the Company filed an amendment to the Company’s amended and restated certificate of incorporation to effect a reverse split of shares of the Company’s common stock and convertible preferred stock on a 4.9175-for-one basis (the “Reverse Stock Split”). The par value and the authorized number of shares of the convertible preferred stock and common stock were not adjusted in connection with the Reverse Stock Split. All references to common stock, convertible preferred stock, warrants to purchase common stock, options to purchase common stock, early exercised options, restricted stock, share data, per share data and related information contained in the accompanying condensed financial statements have been retrospectively adjusted to reflect the effect of the Reverse Stock Split for all periods presented.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the 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 revenue and expenses during the reporting period. Significant estimates and assumptions made in the accompanying condensed financial statements include, but are not limited to, the fair value of common stock, the fair value of stock options, the research period of the collaboration agreement with AbbVie Biotechnology Ltd., operating lease liabilities, income tax uncertainties and certain accruals. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could differ from those estimates.
Segment Reporting	Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the Company’s Chief Operating Decision Maker in making decisions regarding resource allocation and assessing performance. To date, the Company has viewed its operations and manages its business as one segment operating primarily in the United States.
Cash, Cash Equivalents and Restricted Cash	Cash, Cash Equivalents and Restricted Cash The Company considers all highly liquid investments purchased with original maturities of three months or less from the purchase date to be cash equivalents. Cash equivalents consist primarily of amounts invested in money market accounts and are stated at fair value. The Company maintained restricted cash of $0.5 million for both periods ended as of September 30, 2019 and The following table provides a reconciliation of cash and cash equivalents and restricted cash reported within the balance sheets that sum to the total of the same amounts shown in the statement of cash flows. As of September 30, 2019 December 31, 2018 September 30, 2018 (in thousands) Balance Sheets Cash and cash equivalents $ 56,764 $ 89,493 $ 27,858 Restricted cash (included in other assets) 467 467 467 Cash, cash equivalents and restricted cash in Statements of Cash Flows $ 57,231 $ 89,960 $ 28,325
Marketable Securities	Marketable Securities The Company generally invests its excess cash in money market funds and investment grade short- to intermediate-term fixed income securities. Such investments are included in cash and cash equivalents, short-term marketable securities or long-term marketable securities on the balance sheets. Marketable securities with a maturity date greater than 90 days and less than one year at each balance sheet date are classified as short-term. Marketable securities with a maturity date greater than one year at each balance sheet date are classified as long-term. All of the Company’s marketable securities are considered available-for-sale and are reported at fair value with unrealized gains and losses included as a component of stockholders’ equity (deficit). The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity, which is included in interest income, net on the statements of operations. Realized gains and losses and declines in value judged to be other-than-temporary, if any, on marketable securities are included in interest income, net on the statements of operations. The cost of securities sold is determined using specific identification. The Company periodically evaluates whether declines in the fair values of its marketable securities below their book value are other-than-temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss, as well as the Company’s ability and intent to hold the marketable security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the marketable security or it is more likely than not it will be required to sell any marketable securities before recovery of its amortized cost basis. Factors considered include quoted market prices, recent financial results and operating trends, implied values from any recent transactions or offers of investee securities, credit quality of debt instrument issuers, other publicly available information that may affect the value of the marketable security, duration and severity of the decline in value, and the Company’s strategy and intentions for holding the marketable security.
Concentration of Credit Risk	Concentration of Credit Risk Cash, cash equivalents and marketable securities consist of financial instruments that potentially subject the Company to a concentration of credit risk, to the extent of the amounts recorded on the balance sheets. The Company minimizes the amount of credit exposure by investing cash that is not required for immediate operating needs in money market funds and marketable securities.
Leases	Leases The Company evaluates arrangements at inception to determine if an arrangement is or contains a lease. Operating lease assets represent the Company’s right to use an underlying asset for the lease term and operating lease liabilities represent the Company’s obligation to make lease payments arising from the lease. Operating lease assets and liabilities are recognized at the commencement date of the lease based upon the present value of lease payments over the lease term. When determining the lease term, the Company includes options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. The Company uses an incremental borrowing rate that the Company would expect to incur for a fully collateralized loan over a similar term under similar economic conditions to determine the present value of the lease payments. The lease payments used to determine the Company’s operating lease assets may include lease incentives and stated rent increases and are recognized in the Company’s operating lease assets in the condensed balance sheets. Operating lease liabilities are accreted over the term of the lease using the incremental borrowing rate and the associated expense is recorded to operating expenses in the condensed statement of operations and comprehensive loss. Operating lease assets are amortized to rent expense over the lease term and included in operating expenses in the condensed statement of operations and comprehensive loss. Variable lease payments are recognized as the associated obligation is incurred.
Fair Value Measurement	Fair Value Measurement Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability, or an exit price, in the principal or most advantageous market for that asset or liability in an orderly transaction between market participants on the measurement date, and established a fair value hierarchy that requires an entity to maximize the use of observable inputs, where available, and minimize the use of unobservable inputs when measuring fair value. The Company measures fair value based on a three-level hierarchy of inputs, of which the first two are considered observable and the last unobservable. Unobservable inputs reflect the Company’s own assumptions about current market conditions. The three-level hierarchy of inputs is as follows: Level 1—Observable inputs such as unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date; Level 2—Inputs (other than quoted prices included in Level 1) are either directly or indirectly observable for the asset or liability. These include quoted prices for similar assets or liabilities in active markets and quoted prices for identical or similar assets or liabilities in markets that are not active; and Level 3— Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The carrying amounts reflected in the accompanying balance sheets for cash and cash equivalents, short-term marketable securities, prepaid expenses, other current assets, accounts payable, accrued expenses and other current liabilities approximate their fair values due to their short-term nature.
Property and Equipment, Net	Property and Equipment, Net Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation is computed on a straight-line basis over the estimated useful lives of the related assets, generally three to five years. Leasehold improvements are amortized using the straight-line method over the shorter of the assets’ estimated useful lives or the remaining term of the lease. Depreciation and amortization begin at the time the asset is placed in service. Maintenance and repairs are charged to operations as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the balance sheet and the resulting gain or loss is reflected in operations.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company evaluates its long-lived assets, including property and equipment, for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets or group of assets may not be fully recoverable. If indicators of impairment exist and the undiscounted future cash flows that the assets are expected to generate are less than the carrying amount of the assets, the Company reduces the carrying amount of the assets through an impairment charge to their estimated fair values based on a discounted cash flow approach or, when available and appropriate, to comparable market values. There were no impairments of long-lived assets for any of the periods presented.
Revenue Recognition	Revenue Recognition Effective January 1, 2017, the Company early adopted Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers The Company enters into corporate collaborations under which it may obtain upfront license fees, research and development funding, and development, regulatory and commercial milestone payments and royalty payments. The Company’s performance obligations under these arrangements may include licenses of intellectual property, distribution rights, research and development services, delivery of manufactured product and/or participation on joint steering committees. Licenses of intellectual property: If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenue from upfront license fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. For licenses that are bundled with other promises, the Company utilizes judgement to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring proportional performance for purposes of recognizing revenue from non-refundable, up-front fees. The Company evaluates the measure of proportional performance each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Milestone payments: At the inception of each arrangement that includes development, regulatory or commercial milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price. Topic 606 suggests two alternatives to use when estimating the amount of variable consideration: the expected value method and the most likely amount method. Under the expected value method, an entity considers the sum of probability-weighted amounts in a range of possible consideration amounts. Under the most likely amount method, an entity considers the single most likely amount in a range of possible consideration amounts. Whichever method is used, it should be consistently applied throughout the life of the contract; however, it is not necessary for the Company to use the same approach for all contracts. The Company expects to use the most likely amount method for development and regulatory milestone payments. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s or the licensee’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The transaction price is then allocated to each performance obligation on a relative stand-alone selling price basis. The Company recognizes revenue as or when the performance obligations under the contract are satisfied. At the end of each subsequent reporting period, the Company re-evaluates the probability or achievement of each such milestone and any related constraint, and if necessary, adjusts its estimates of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. Commercial milestones and royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and in which the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue when the related sales occur. To date, the Company has not recognized any royalty revenue resulting from its collaboration arrangements. Upfront payments and fees are recorded as deferred revenue upon receipt or when due, and may require deferral of revenue recognition to a future period until the Company performs its obligations under these arrangements. Amounts due to the Company are recorded as accounts receivable when the Company’s right to consideration is unconditional.
Research and Development Expenses and Accrued Research and Development Costs	Research and Development Expenses and Accrued Research and Development Costs The Company expenses research and development costs as incurred. Research and development expenses consist of personnel costs for the Company’s research and product development employees. Also included are non-personnel costs such as professional fees payable to third parties for preclinical studies, clinical trials, research services, laboratory supplies and equipment maintenance and depreciation, intellectual property licenses and other consulting costs. The Company estimates preclinical and clinical study and research expenses based on the services performed, pursuant to contracts with research institutions that conduct and manage preclinical studies, clinical trials and research services on its behalf. The Company estimates these expenses based on discussions with internal management personnel and external service providers as to the progress or stage of completion of services and the contracted fees to be paid for such services. The Company records the estimated costs of research and development activities based upon the estimated amount services provided but not yet invoiced, and includes these costs in development expenses. The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its third-party service provides under the service agreements. The Company makes significant judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, the Company adjusts its accrued liabilities. The Company has not experienced any material differences between accrued costs and actual costs incurred. However, the status and timing of actual services performed may vary from the Company’s estimates, resulting in adjustments to expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect the Company’s results of operations. Payments associated with licensing agreements to acquire exclusive license to develop, use, manufacture and commercialize products that have not reached technological feasibility and do not have alternate future use are expensed as incurred. Payments made to third parties under these arrangements in advance of the performance of the related services by the third parties are recorded as prepaid expenses until the services are rendered. Such payments are evaluated for current or long-term classification based on when such services are expected to be received.
Stock-Based Compensation	Stock-Based Compensation The Company maintains a stock-based compensation plan as a long-term incentive for employees, consultants and members of the Company’s board of directors (the “Board”). The plan allows for the issuance of non-statutory options (“NSOs”) and incentive stock options to employees and NSOs to nonemployees. Share-based payments are measured using fair-value-based measurements and recognized as compensation expense over the service period in which the awards are expected to vest. The Company’s fair-value-based measurements of awards to employees and directors as of the grant date utilize the single-option award-valuation approach, and the Company uses the straight-line method for expense attribution. The fair-value-based measurements of options granted to nonemployees are remeasured at each period end until the options vest and are amortized to expense as earned. The valuation model used for calculating the estimated fair value of stock awards is the Black-Scholes option-pricing model. The Black-Scholes model requires the Company to make assumptions and judgments about the variables used in the calculations, including the expected term (weighted-average period of time that the options granted are expected to be outstanding), the expected volatility of the Company’s common stock, the related risk-free interest rate and the expected dividend. The Company has elected to recognize forfeitures of share-based payment awards as they occur. Effective January 1, 2018, the Company early adopted ASU No. 2018-07, Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting
Convertible Preferred Stock	Convertible Preferred Stock The Company records all shares of convertible preferred stock at their respective fair values less issuance costs on the dates of issuance. The convertible preferred stock is recorded outside of stockholders’ deficit because, in the event of certain deemed liquidation events considered not solely within the Company’s control, such as a merger, acquisition and sale of all or substantially all of the Company’s assets, the convertible preferred stock will become redeemable at the option of the holders. In the event of a change of control of the Company, proceeds received from the sale of such shares will be distributed in accordance with the liquidation preferences set forth in the Company’s Amended and Restated Certificate of Incorporation unless the holders of convertible preferred stock have converted their shares of convertible preferred stock into shares of common stock. The Company has determined not to adjust the carrying values of the convertible preferred stock to the liquidation preferences of such shares because of the uncertainty of whether or when such an event would occur.
Income Taxes	Income Taxes Income taxes are accounted for under the asset and liability method. Under this method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates applied to taxable income in the years in which those temporary differences are expected to be realized. The effect on deferred tax assets and liabilities of a change in tax rates is recognized as income in the period that includes the enactment date. A valuation allowance is established when necessary to reduce deferred tax assets to the amount expected to be realized. Financial statement effects of uncertain tax positions are recognized when it is more-likely-than-not, based on the technical merits of the position, that it will be sustained upon examination. Interest and penalties related to unrecognized tax benefits are included as a component of other expense. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits. The Company accounts for uncertain tax positions in accordance with ASC 740-10, Accounting for Uncertainty in Income Taxes The Company includes any penalties and interest expense related to income taxes as a component of provision for income tax as necessary. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration for potentially dilutive securities. As discussed in Note 9, the unvested portion of early exercised stock options are excluded from the computation of weighted average shares as the continuing vesting of such shares is contingent on the holders’ continued service to the Company. Diluted net loss per share is the same as basic net loss per share for each period presented, since the effects of potentially dilutive securities are antidilutive given the net loss of the Company.
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive loss includes net loss and certain changes in stockholders’ equity (deficit) that are excluded from net loss, primarily unrealized gains or losses on the Company’s marketable securities.
Deferred Offering Costs	Deferred Offering Costs The Company had deferred offering costs consisting of legal, accounting and other fees and costs directly attributable to the IPO, which was completed in February 2019. The deferred offering costs were offset against the gross proceeds of the IPO. As of September 30, 2019 and December 31, 2018, zero and $3.0 million of deferred offering costs were included in other assets on the balance sheets, respectively.
Emerging Growth Company Status	Emerging Growth Company Status The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 (the “JOBS Act”). Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act until such time as those standards apply to private companies. The Company has elected to use this extended transition period for complying with new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that the Company (i) is no longer an emerging growth company or (ii) affirmatively and irrevocably opts out of the extended transition period provided in the JOBS Act. As a result, the accompanying condensed financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates. As described in “Recently Adopted Accounting Pronouncements” below, the Company early adopted Revenue from Contracts with Customers Stock Compensation—Improvements to Employee Share-Based Payment Accounting Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting, and (Topic 842), Leases as the JOBS Act does not preclude an emerging growth company from adopting a new or revised accounting standard earlier than the time that such standard applies to private companies. The Company expects to use the extended transition period for any other new or revised accounting standards during the period in which it remains an emerging growth company.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2016-02 (Topic 842), Leases (“ASU 2016-02”) In February 2018, the FASB issued ASU No. 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments ASU 2018-19 ASU 2019-04 In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Schedule of Reconciliation of Cash and Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash and cash equivalents and restricted cash reported within the balance sheets that sum to the total of the same amounts shown in the statement of cash flows. As of September 30, 2019 December 31, 2018 September 30, 2018 (in thousands) Balance Sheets Cash and cash equivalents $ 56,764 $ 89,493 $ 27,858 Restricted cash (included in other assets) 467 467 467 Cash, cash equivalents and restricted cash in Statements of Cash Flows $ 57,231 $ 89,960 $ 28,325

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Summary of Financial Assets Measured at Fair Value and Fair Value Hierarchy of Valuation	The following table presents information about the Company’s financial assets that are measured at fair value and indicates the fair value hierarchy of the valuation: Fair Value Measurements at September 30, 2019 Total Level 1 Level 2 Level 3 (in thousands) Assets Cash Equivalents: Money market funds $ 39,879 $ 39,879 $ — $ — U.S. government treasuries 14,979 14,979 — — Short-term marketable securities U.S. government treasuries 27,955 27,955 — — Corporate debt securities 19,010 — 19,010 — U.S. government agency securities 17,498 — 17,498 — Total assets $ 119,321 $ 82,813 $ 36,508 $ — Fair Value Measurements at December 31, 2018 Total Level 1 Level 2 Level 3 (in thousands) Assets Cash Equivalents: Money market funds $ 60,396 60,396 $ — $ — Total assets $ 60,396 $ 60,396 $ — $ —

Available-for-Sale Securities (

Available-for-Sale Securities (Tables)	9 Months Ended
	Sep. 30, 2019
Investments Debt And Equity Securities [Abstract]
Schedule of Available-for-Sale Securities at Fair Value	All marketable securities were considered available-for-sale at September 30, 2019. The amortized cost, gross unrealized holding gains or losses, and fair value of the Company’s marketable securities by major security type at each balance sheet date are summarized in the tables below : September 30, 2019 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value (in thousands) Cash equivalents U.S. government treasuries $ 14,977 $ 2 $ — $ 14,979 Money market funds 23,782 — — 23,782 Total cash equivalents 38,759 2 — 38,761 Short-term marketable securities: U.S. government treasuries 27,944 11 — 27,955 U.S. government agency securities 17,476 21 — 17,497 Corporate debt securities 18,960 51 — 19,011 Total short-term marketable securities 64,380 83 — 64,463 Total $ 103,139 $ 85 $ — $ 103,224 December 31, 2018 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value (in thousands) Cash equivalents Money market funds $ 60,396 $ — $ — $ 60,396 Total cash equivalents $ 60,396 $ — $ — $ 60,396

Balance Sheet Components (Table

Balance Sheet Components (Tables)	9 Months Ended
	Sep. 30, 2019
Balance Sheet Related Disclosures [Abstract]
Property and Equipment, Net	Property and equipment, net consists of the following: September 30, December 31, 2019 2018 (in thousands) Laboratory equipment $ 3,679 $ 2,402 Furniture and fixtures 552 312 Computer equipment and software 91 32 Leasehold improvements 8,862 360 Construction in progress — 906 13,184 4,012 Less: Accumulated depreciation and amortization (1,414 ) (1,014 ) Total property and equipment, net $ 11,770 $ 2,998
Accrued Liabilities	Accrued liabilities consist of the following: September 30, December 31, 2019 2018 Accrued research and development $ 2,016 $ 504 Accrued personnel costs 1,695 1,593 Accrued professional and consulting fees 2,663 333 Accrued offering costs — 709 Other 1,345 202 Total accrued liabilities $ 7,719 $ 3,341

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2019
Commitments And Contingencies Disclosure [Abstract]
Summary of Operating Leases in Condensed Balance Sheets	The following table summarizes the presentation in the Company’s condensed balance sheets of its operating lease (in thousands): As of September 30, 2019 Assets: Operating lease right-of-use assets $ 7,106 Liabilities Operating lease liabilities $ 987 Operating lease liabilities, net of current portion 14,051 Total operating lease liabilities $ 15,038
Schedule of Future Minimum Lease Payments Under Tizona Lease and Cove Lease	F uture minimum lease payments under the Cove Lease as of September 30, 2019 are as follows (in thousands): As of September 30, 2019 Operating Lease Commitments 2019 $ 427 2020 2,487 2021 2,566 2022 2,647 2023 2,731 Thereafter 10,251 Total future minimum lease payments 21,109 Less: Present value adjustment for minimum lease commitments (6,071 ) Total $ 15,038
Schedule of Future Minimum Lease Payments Under Operating Leases	The future minimum lease payments under the Company’s operating leases as of December 31, 2018, were as follows (in thousands): As of December 31, 2018 Operating Lease Commitments 2019 $ 1,308 2020 2,762 2021 2,566 2022 2,647 2023 2,731 Thereafter 10,251 Total $ 22,265

Equity (Tables)

Equity (Tables)	9 Months Ended
	Sep. 30, 2019
Equity [Abstract]
Summary of Option Activity Under Equity Incentive Plan	The following summarizes option activity under the 2019 Plan: Weighted Weighted Average Number of Average Remaining Aggregate Outstanding Exercise Contractual Intrinsic Options Price Life Value (in years) (in thousands) Balance as of December 31, 2018 3,323,988 1.61 9.07 $ 1,675 Options granted 302,412 13.20 Options exercised (280,044 ) 0.89 Options cancelled (83,864 ) 1.32 Balance as of September 30, 2019 3,262,492 2.75 8.52 $ 35,661 Vested and expected to vest as of September 30, 2019 3,262,492 2.75 8.52 $ 35,661 Exercisable as of September 30, 2019 684,569 1.61 7.63 $ 8,254
Summary of Fair Value of Employee and Director Stock Option Awards using a Black-Scholes Option-Pricing Model	The fair value of employee and director stock option awards was estimated at the date of grant using a Black-Scholes option-pricing model with the following weighted-average assumptions: Nine Months Ended September 30, 2019 2018 Expected term (years) 5.86 6.08 Expected volatility 77.76 % 75.03 % Risk-free interest rate 2.22 % 2.80 % Expected dividend 0 % 0 %
Summary of Stock-based Compensation	Total stock-based compensation was as follows: Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 (in thousands) Operating Expenses Research and development $ 169 $ 71 $ 466 $ 211 General and administrative 423 103 962 250 Total stock-based compensation $ 592 $ 174 $ 1,428 $ 461
Summary of Restricted Stock Activity	A summary of restricted stock activity is shown in the below table: Number of of Restricted Stock Outstanding Restricted shares- December 31, 2018 22,178 Restricted stock awards vested (22,178 ) Unvested shares repurchased — Restricted shares- September 30, 2019 —

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
Summary of Outstanding Potentially Dilutive Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share	The following outstanding potentially dilutive common stock equivalents have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect: As of September 30, As of December 31, 2019 2018 Convertible preferred stock (as converted) — 16,618,448 Common stock options issued and outstanding 3,262,492 3,323,988 Restricted Stock subject to future vesting — 22,178 Early exercised stock options subject to future vesting 113,856 149,565 Warrants to purchase shares of common stock — 565,270 Total 3,376,348 20,679,449
Summary of Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders	The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company (in thousands, except per share data): Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Net loss $ (15,908 ) $ (6,759 ) $ (41,304 ) $ (17,634 ) Net loss per share, basic and diluted $ (0.65 ) $ (6.23 ) $ (1.95 ) $ (17.22 ) Weighted-average shares used to compute basic and diluted net loss per share 24,457,402 1,084,477 21,202,848 1,024,105

Organization - Additional Infor

Organization - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Feb. 07, 2019	Sep. 30, 2019	Mar. 14, 2019	Dec. 31, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Month and year of incorporation		2015-03
Common stock shares sold		24,613,803		1,383,221
Gross proceeds from issuance of common stock		$ 70,647
Accumulated deficit		(103,895)		$ (62,591)
Cash, cash equivalents and marketable securities		$ 121,200
Redeemable Convertible Preferred Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Convertible shares of common stock	17,181,491
Reclassified from temporary equity to additional paid in capital	$ 129,600
Convertible preferred stock, outstanding			0
IPO
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Public offering price per share	$ 14
Common stock shares sold	5,769,201
Gross proceeds from issuance of common stock	$ 80,800
Net proceeds after deducting underwriting discounts, commissions and offering costs	$ 70,700

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details)	Jan. 28, 2019	Sep. 30, 2019USD ($)Segment	Jan. 01, 2019USD ($)	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)
Summary Of Significant Accounting Policy [Line Items]
Highly liquid investments purchased with maximum original maturities		3 months
Restricted cash		$ 467,000		$ 467,000	$ 467,000
Impairment of long-lived assets		0
Interest or penalties charged related to unrecognized tax benefits		0
Deferred offering costs		0		$ 3,000,000
Operating lease right-of-use asset		7,106,000
Operating lease liabilities		15,038,000
ASC 740-10
Summary Of Significant Accounting Policy [Line Items]
Interest or penalties charged related to unrecognized tax benefits		$ 0
ASU 2016-02
Summary Of Significant Accounting Policy [Line Items]
Operating lease right-of-use asset			$ 8,400,000
Operating lease liabilities			$ 15,100,000
Minimum
Summary Of Significant Accounting Policy [Line Items]
Property and equipment, estimated useful life		3 years
Maximum
Summary Of Significant Accounting Policy [Line Items]
Property and equipment, estimated useful life		5 years
United States
Summary Of Significant Accounting Policy [Line Items]
Number of operating segments | Segment		1
Common Stock | Convertible Preferred Stock
Summary Of Significant Accounting Policy [Line Items]
Reverse split of shares	4.9175
Reverse split of shares, description		4.9175-for-one basis

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash and Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Cash and cash equivalents	$ 56,764	$ 89,493	$ 27,858
Restricted cash (included in other assets)	467	467	467
Cash, cash equivalents and restricted cash in Statements of Cash Flows	$ 57,231	$ 89,960	$ 28,325	$ 29,423

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value and Fair Value Hierarchy of Valuation (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets
Assets fair value	$ 119,321	$ 60,396
Cash Equivalents | Money Market Funds
Assets
Assets fair value	39,879	60,396
Cash Equivalents | U.S. government treasuries
Assets
Assets fair value	14,979
Short-term marketable securities | U.S. government treasuries
Assets
Assets fair value	27,955
Short-term marketable securities | Corporate debt securities
Assets
Assets fair value	19,010
Short-term marketable securities | U.S. government agency securities
Assets
Assets fair value	17,498
Level 1
Assets
Assets fair value	82,813	60,396
Level 1 | Cash Equivalents | Money Market Funds
Assets
Assets fair value	39,879	60,396
Level 1 | Cash Equivalents | U.S. government treasuries
Assets
Assets fair value	14,979
Level 1 | Short-term marketable securities | U.S. government treasuries
Assets
Assets fair value	27,955
Level 2
Assets
Assets fair value	36,508	0
Level 2 | Short-term marketable securities | Corporate debt securities
Assets
Assets fair value	19,010
Level 2 | Short-term marketable securities | U.S. government agency securities
Assets
Assets fair value	17,498
Level 3
Assets
Assets fair value	$ 0	$ 0

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value, assets	$ 119,321,000	$ 60,396,000
Fair value, assets, level 1 to level 2 transfers, amount	0	0
Financial liabilities fair value measurements	0	0
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value, assets	0	0
Fair value, liabilities	$ 0	$ 0

Available-for-Sale Securities -

Available-for-Sale Securities - Schedule of Available-for-Sale Securities at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 103,139
Gross Unrealized Gain	85
Fair Value	103,224
Cash Equivalents
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	38,759	$ 60,396
Gross Unrealized Gain	2
Fair Value	38,761	60,396
Cash Equivalents | U.S. government treasuries
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	14,977
Gross Unrealized Gain	2
Fair Value	14,979
Cash Equivalents | Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	23,782	60,396
Fair Value	23,782	$ 60,396
Short-term marketable securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	64,380
Gross Unrealized Gain	83
Fair Value	64,463
Short-term marketable securities | U.S. government treasuries
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	27,944
Gross Unrealized Gain	11
Fair Value	27,955
Short-term marketable securities | U.S. government agency securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	17,476
Gross Unrealized Gain	21
Fair Value	17,497
Short-term marketable securities | Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	18,960
Gross Unrealized Gain	51
Fair Value	$ 19,011

Available-for-Sale Securities_2

Available-for-Sale Securities - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Marketable securities in unrealized loss position	$ 0	$ 0
Maximum
Schedule Of Available For Sale Securities [Line Items]
Marketable securities maturity period	2 years

Balance Sheet Components - Prop

Balance Sheet Components - Property and Equipment, Net (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment	$ 13,184	$ 4,012
Less: Accumulated depreciation and amortization	(1,414)	(1,014)
Total property and equipment, net	11,770	2,998
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment	3,679	2,402
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment	552	312
Computer Equipment and Software
Property Plant And Equipment [Line Items]
Property and equipment	91	32
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment	$ 8,862	360
Construction in Progress
Property Plant And Equipment [Line Items]
Property and equipment		$ 906

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Balance Sheet Related Disclosures [Abstract]
Depreciation and amortization expense	$ 1.1	$ 0.2	$ 0.9	$ 0.4

Balance Sheet Components - Accr

Balance Sheet Components - Accrued Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Balance Sheet Related Disclosures [Abstract]
Accrued research and development	$ 2,016	$ 504
Accrued personnel costs	1,695	1,593
Accrued professional and consulting fees	2,663	333
Accrued offering costs		709
Other	1,345	202
Total accrued liabilities	$ 7,719	$ 3,341

Convertible Notes - Additional

Convertible Notes - Additional Information (Details) - $ / shares	1 Months Ended	12 Months Ended
	May 31, 2017	Dec. 31, 2018	Sep. 30, 2019	Feb. 12, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Convertible Preferred Stock
Convertible Notes [Line Items]
Convertible preferred stock		16,618,448	0		10,118,513	6,989,973	6,989,973	6,989,973
2016 Notes and 2017 Notes
Convertible Notes [Line Items]
Warrants exercisable		565,270
Class of Warrant or Right, Outstanding		565,270
2016 Notes and 2017 Notes | IPO
Convertible Notes [Line Items]
Warrants exercise price				$ 14
2016 Notes and 2017 Notes | IPO | Warrant
Convertible Notes [Line Items]
Warrants converted into shares of common stock		563,043
2016 Notes and 2017 Notes | Series B | Convertible Preferred Stock
Convertible Notes [Line Items]
Accrued interest converted into shares	811,103

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	1 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 30, 2019	Dec. 31, 2018	Aug. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Feb. 28, 2017
Commitments And Contingencies [Line Items]
Weighted average remaining lease term				7 years 9 months 18 days		7 years 9 months 18 days
Variable lease costs				$ 0.1		$ 0.3
Weighted average incremental borrowing rate				8.95%		8.95%
Rent expense				$ 0.9	$ 0.3	$ 2.2	$ 0.8
Amortization of right-of-use lease assets				0.3	$ 0	1.1	$ 0
Rent expense under ASC 840								$ 1.2
Office and Laboratory Space
Commitments And Contingencies [Line Items]
Lease term			8 years
Lease term option to extend period			8 years
Lease commencement date			Jul. 1, 2019
Tenant improvement allowance entitled			$ 5.2
Additional tenant improvement allowance			$ 1.4
Additional tenant improvement allowance exercised		$ 1.4
Tenant improvement allowance utilized under lease recorded as construction in progress				$ 6.6		$ 6.6
Office Space
Commitments And Contingencies [Line Items]
Lease term option to extend period				8 years		8 years
Weighted average remaining lease term				8 years		8 years
Tizona Sublease Termination Agreement
Commitments And Contingencies [Line Items]
Lease termination fees	$ 0.4
Tizona Therapeutics, Inc.
Commitments And Contingencies [Line Items]
Lease term									36 months

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Operating Leases in Condensed Balance Sheets (Details) $ in Thousands	Sep. 30, 2019USD ($)
Assets
Operating lease right-of-use asset	$ 7,106
Liabilities
Operating lease liabilities	987
Operating lease liabilities, net of current portion	14,051
Total operating lease liabilities	$ 15,038

Commitments and Contingencies_3

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Under Tizona Lease and Cove Lease (Details) $ in Thousands	Sep. 30, 2019USD ($)
Commitments And Contingencies Disclosure [Abstract]
2019	$ 427
2020	2,487
2021	2,566
2022	2,647
2023	2,731
Thereafter	10,251
Total future minimum lease payments	21,109
Less: Present value adjustment for minimum lease commitments	(6,071)
Operating lease liabilities	$ 15,038

Commitments and Contingencies_4

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Under Operating Leases (Details) $ in Thousands	Dec. 31, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
2019	$ 1,308
2020	2,762
2021	2,566
2022	2,647
2023	2,731
Thereafter	10,251
Total	$ 22,265

Collaboration & License Agree_2

Collaboration & License Agreements - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, noncurrent		$ 4,420,000		$ 4,420,000		$ 7,792,000
Deferred revenue, current		4,080,000		$ 4,080,000		$ 4,250,000
Collaborative Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration termination notice period				90 days
Total revenue				$ 3,500,000	$ 3,700,000
AbbVie Inc. | Development, Regulatory, and Commercial Sale Milestones For Licensed Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement upfront payment amount.		17,000,000		17,000,000
AbbVie Inc. | Development, Regulatory, and Commercial Sale Milestones For Licensed Products | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement upfront payment amount.		600,000,000		$ 600,000,000
AbbVie Inc. | Collaborative Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration termination notice period				30 days
Transaction price				$ 17,000,000
Total revenue		1,100,000	$ 1,400,000
Deferred revenue		8,500,000		8,500,000
Deferred revenue, noncurrent		4,400,000		4,400,000
Deferred revenue, current		4,100,000		4,100,000
Milestone payments under collaboration agreement				0
Recognize royalty revenue		0		0
AbbVie Inc. | Research and Development Activities
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement upfront payment amount.		17,000,000		$ 17,000,000
Estimated period to recognize upfront payment				4 years
Werewolf Therapeutics, Inc. | Licensing Agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Recognize royalty revenue		$ 0	$ 0	$ 0	$ 0
Ownership interest in the company's capital stock	5.00%
Upfront fee received	$ 500,000			$ 500,000

Equity - Additional Information

Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2015	Jun. 30, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Stock-based compensation expense	$ 592	$ 174		$ 1,428	$ 461
Non-employee Awards
Class Of Stock [Line Items]
Stock-based compensation expense	0	0		100	100
Early Exercised Stock Options - 2015 Plan
Class Of Stock [Line Items]
Other current liabilities relating to shares subject to repurchase	$ 161,000		$ 161,000	$ 161,000				$ 188,000
Early exercised shares repurchased	113,856		113,856	113,856				149,565
2019 Equity Incentive Plan
Class Of Stock [Line Items]
Shares reserved for future issuance	2,113,843		2,113,843	2,113,843
Number of shares authorized for grant							2,340,438
Aggregate intrinsic value of options exercisable	$ 700	$ 100	$ 3,100		100
Future tax benefit related to options exercised			$ 0		$ 0
2019 Equity Incentive Plan | Employee Stock Option
Class Of Stock [Line Items]
Weighted average grant date fair value of options vested	$ 0.86	$ 0.71	$ 0.86		$ 0.71
Weighted average grant date fair value of options granted	$ 9.80	$ 1.20	$ 9.29		$ 1.18
Unrecognized stock based compensation expense	$ 4,400		$ 4,400	$ 4,400
Weighted-average period for unrecognized compensation cost to recognize	2 years 8 months 12 days
2015 Plan
Class Of Stock [Line Items]
Share-based compensation purchase price of restricted stock awards						$ 0.0005

Equity - Summary of Option Acti

Equity - Summary of Option Activity Under Equity Incentive Plan (Details) - 2019 Equity Incentive Plan $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares
Number of Outstanding Options
Outstanding Options, Beginning Balance | shares	3,323,988
Options granted | shares	302,412
Options exercised | shares	(280,044)
Options cancelled | shares	(83,864)
Outstanding Options, Ending Balance | shares	3,262,492	3,323,988
Vested and expected to vest as of September 30, 2019 | shares	3,262,492
Exercisable as of September 30, 2019 | shares	684,569
Weighted Average Exercise Price
Outstanding Options, Weighted Average Exercise Price | $ / shares	$ 1.61
Options granted, Weighted Average Exercise Price | $ / shares	13.20
Options exercised, Weighted Average Exercise Price | $ / shares	0.89
Options cancelled, Weighted Average Exercise Price | $ / shares	1.32
Outstanding Options, Weighted Average Exercise Price | $ / shares	2.75	$ 1.61
Vested and expected to vest as of September 30, 2019 | $ / shares	2.75
Exercisable as of September 30, 2019 | $ / shares	$ 1.61
Weighted Average Remaining Contractual Life (in years)
Outstanding Options, Weighted Average Remaining Contractual Life	8 years 6 months 7 days	9 years 25 days
Vested and expected to vest as of September 30, 2019	8 years 6 months 7 days
Exercisable as of September 30, 2019	7 years 7 months 17 days
Aggregate Intrinsic Value
Options Outstanding, Aggregate Intrinsic Value | $	$ 1,675
Options Outstanding, Aggregate Intrinsic Value | $	35,661	$ 1,675
Vested and expected to vest as of September 30, 2019 | $	35,661
Exercisable as of September 30, 2019 | $	$ 8,254

Equity - Summary of Fair Value

Equity - Summary of Fair Value of Employee and Director Stock Option Awards using a Black-Scholes Option-Pricing Model (Details) - Employee Stock Option	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	5 years 10 months 9 days	6 years 9 months 18 days
Expected volatility	77.76%	75.03%
Risk-free interest rate	2.22%	2.80%
Expected dividend	0.00%	0.00%

Equity - Total Stock-based Comp

Equity - Total Stock-based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	$ 592	$ 174	$ 1,428	$ 461
Research and development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	169	71	466	211
General and administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	$ 423	$ 103	$ 962	$ 250

Equity - Summary of Restricted

Equity - Summary of Restricted Stock Activity (Details) - Restricted Stock	9 Months Ended
	Sep. 30, 2019shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted shares, Beginning Balance	22,178
Restricted stock awards vested	(22,178)

Net Loss Per Share - Summary of

Net Loss Per Share - Summary of Outstanding Potentially Dilutive Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Outstanding potentially dilutive common stock equivalents	3,376,348	20,679,449
Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Outstanding potentially dilutive common stock equivalents		16,618,448
Common Stock Options Issued and Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Outstanding potentially dilutive common stock equivalents	3,262,492	3,323,988
Restricted Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Outstanding potentially dilutive common stock equivalents		22,178
Early Exercised Stock Options Subject To Future Vesting
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Outstanding potentially dilutive common stock equivalents	113,856	149,565
Warrants To Purchase Shares Of Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Outstanding potentially dilutive common stock equivalents		565,270

Net Loss Per Share - Summary _2

Net Loss Per Share - Summary of Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net loss	$ (15,908)	$ (11,817)	$ (13,579)	$ (6,759)	$ (5,994)	$ (4,881)	$ (41,304)	$ (17,634)
Net loss per share, basic and diluted	$ (0.65)			$ (6.23)			$ (1.95)	$ (17.22)
Weighted-average common shares used in computing net loss per share, basic and diluted	24,457,402			1,084,477			21,202,848	1,024,105