Kronos Bio (KRON) 10-Q 18 Nov 20 2020 Q3 Quarterly report Financial data

Cover Page

Cover Page - shares	9 Months Ended
	Sep. 30, 2020	Nov. 16, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2020
Document Transition Report	false
Entity File Number	001-39592
Entity Registrant Name	Kronos Bio, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	82-1895605
Entity Address, Address Line One	1300 So. El Camino Real
Entity Address, Address Line Two	Suite 300
Entity Address, City or Town	San Mateo
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94402
City Area Code	650
Local Phone Number	781-5200
Title of 12(b) Security	Common stock, $0.001 par value per share
Trading Symbol	KRON
Security Exchange Name	NASDAQ
Entity Current Reporting Status	No
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		56,033,504
Entity Central Index Key	0001741830
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Amendment Flag	false

Condensed Balance Sheets (Unaud

Condensed Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	[1]
Current assets:
Cash and cash equivalents	$ 156,196	$ 32,570
Short-term investments	55,069	59,614
Prepaid expenses and other current assets	1,628	1,119
Total current assets	212,893	93,303
Long-term investments	4,774	4,762
Total property and equipment, net	10,301	3,721
Operating lease right-of-use assets	29,355	473
Restricted cash	2,026	0
Other noncurrent assets	3,207	427
Total assets	262,556	102,686
Current liabilities:
Accounts payable	4,443	1,506
Accrued expenses	1,780	818
Convertible notes payable, at fair value	173,944	0
Current portion of operating lease liabilities	1,300	285
Current portion of other liabilities	2,137	88
Total current liabilities	183,604	2,697
Noncurrent operating lease liabilities	29,484	211
Other noncurrent liabilities	2,841	74
Total liabilities	215,929	2,982
Commitments and contingencies (Note 13)
Convertible preferred stock, $0.001 par value; 21,506,977 and 21,506,977 shares authorized as of September 30, 2020 and December 31, 2019, respectively; 21,504,893 and 21,504,893 share issued and outstanding as of September, 2020 and December 31, 2019, respectively; $123,016 and $123,016 liquidation preference as of September 30, 2020 and December 31, 2019, respectively.	122,907	122,907
Stockholder's equity:
Common stock, $0.001 par value, 40,000,000 and 40,000,000 shares authorized as of September 30, 2020 and December 31, 2019 respectively; 6,284,326 and 5,660,391 shares issued and outstanding as of September 30, 2020 and December 31, 2019, respectively.	6	6
Additional paid-in capital	2,416	271
Accumulated deficit	(78,749)	(23,462)
Accumulated other comprehensive income (loss)	47	(18)
Total stockholders' deficit	(76,280)	(23,203)
Total liabilities, convertible preferred stock and stockholders’ deficit	$ 262,556	$ 102,686
[1]	The balance sheet as of December 31, 2019 is derived from the audited financial statements as of that date.

Condensed Balance Sheets (Una_2

Condensed Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jul. 01, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Convertible preferred stock, par value (in dollars per share)	$ 0.001			$ 0.001	$ 7.67		$ 7.67
Convertible preferred stock, shares authorized (in shares)	21,506,977			21,506,977
Convertible preferred stock, shares outstanding (in shares)	21,504,893	21,504,893	21,504,893	21,504,893	21,504,893		14,949,465	7,806,977	7,806,977
Convertible preferred stock, shares issued (in shares)	21,504,893			21,504,893
Convertible preferred stock, liquidation preference	$ 123,016			$ 123,016
Common stock, par value (in dollars per share)	$ 0.001			$ 0.001		$ 0.001
Common stock, shares authorized (in shares)	40,000,000			40,000,000		40,000,000
Common stock, shares issued (in shares)	6,284,326			5,660,391
Common stock, shares outstanding (in shares)	6,284,326			5,660,391

Condensed Statements of Operati

Condensed Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Operating expenses:
Research and development	$ 16,755	$ 3,526	$ 30,125	$ 8,698
General and administrative	4,054	797	6,831	2,262
Total operating expenses	20,809	4,323	36,956	10,960
Loss from operations	(20,809)	(4,323)	(36,956)	(10,960)
Other Income and Expenses [Abstract]
Change in fair value of convertible notes payable	(15,215)	0	(15,215)	0
Interest expense	(3,889)	(1)	(3,890)	(3)
Interest and other income, net	200	311	774	311
Total other income (expense), net	(18,904)	310	(18,331)	308
Net loss	(39,713)	(4,013)	(55,287)	(10,652)
Other comprehensive income (loss):
Net unrealized gain (loss) on available-for-sale securities	(117)	0	65	0
Net comprehensive loss	$ (39,830)	$ (4,013)	$ (55,222)	$ (10,652)
Net loss per share, basic and diluted (in dollars per share)	$ (6.48)	$ (0.74)	$ (9.39)	$ (2.05)
Weighted-average shares of common stock, basic and diluted (in shares)	6,127,146	5,409,433	5,886,191	5,196,681

Condensed Statements of Convert

Condensed Statements of Convertible Preferred Stock and Stockholders' Deficit (Unaudited) - USD ($) $ in Thousands		Total		Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Convertible preferred stock, beginning balance (in shares) at Dec. 31, 2018		7,806,977
Convertible preferred stock, beginning balance at Dec. 31, 2018		$ 17,985
Convertible preferred stock, ending balance (in shares) at Mar. 31, 2019		7,806,977
Convertible preferred stock, ending balance at Mar. 31, 2019		$ 17,985
Common stock, beginning balance (in shares) at Dec. 31, 2018				4,966,227
Beginning balance at Dec. 31, 2018		(7,296)		$ 5	$ 44	$ 0	$ (7,345)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon vesting and exercise of options and vesting of restricted stock (in shares)				29,704
Issuance of common stock upon vesting and exercise of options and vesting of restricted stock		1			1
Stock-based compensation expense		16			16
Net loss		(3,130)					(3,130)
Common stock, ending balance (in shares) at Mar. 31, 2019				4,995,931
Beginning balance at Mar. 31, 2019		$ (10,409)		$ 5	61	0	(10,475)
Convertible preferred stock, beginning balance (in shares) at Dec. 31, 2018		7,806,977
Convertible preferred stock, beginning balance at Dec. 31, 2018		$ 17,985
Convertible preferred stock, ending balance (in shares) at Sep. 30, 2019		21,504,893
Convertible preferred stock, ending balance at Sep. 30, 2019		$ 122,907
Common stock, beginning balance (in shares) at Dec. 31, 2018				4,966,227
Beginning balance at Dec. 31, 2018		(7,296)		$ 5	44	0	(7,345)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net unrealized gain (loss) on available-for-sale securities		0
Net loss		(10,652)
Common stock, ending balance (in shares) at Sep. 30, 2019				5,488,261
Beginning balance at Sep. 30, 2019		$ (17,831)		$ 5	161	0	(17,997)
Convertible preferred stock, beginning balance (in shares) at Mar. 31, 2019		7,806,977
Convertible preferred stock, beginning balance at Mar. 31, 2019		$ 17,985
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of Series A convertible preferred stock, net of issuance costs (in shares)		7,142,488
Issuance of Series A convertible preferred stock, net of issuance costs		$ 54,680
Convertible preferred stock, ending balance (in shares) at Jun. 30, 2019		14,949,465
Convertible preferred stock, ending balance at Jun. 30, 2019		$ 72,665
Common stock, beginning balance (in shares) at Mar. 31, 2019				4,995,931
Beginning balance at Mar. 31, 2019		(10,409)		$ 5	61	0	(10,475)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon vesting and exercise of options and vesting of restricted stock (in shares)				386,239
Issuance of common stock upon vesting and exercise of options and vesting of restricted stock		45			45
Stock-based compensation expense		21			21
Net loss		(3,509)					(3,509)
Common stock, ending balance (in shares) at Jun. 30, 2019				5,382,170
Beginning balance at Jun. 30, 2019		$ (13,852)		$ 5	127	0	(13,984)
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of Series A convertible preferred stock, net of issuance costs (in shares)		6,555,428
Issuance of Series A convertible preferred stock, net of issuance costs		$ 50,242
Convertible preferred stock, ending balance (in shares) at Sep. 30, 2019		21,504,893
Convertible preferred stock, ending balance at Sep. 30, 2019		$ 122,907
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon vesting and exercise of options and vesting of restricted stock (in shares)				106,091
Issuance of common stock upon vesting and exercise of options and vesting of restricted stock		11			11
Stock-based compensation expense		23			23
Net unrealized gain (loss) on available-for-sale securities		0
Net loss		(4,013)					(4,013)
Common stock, ending balance (in shares) at Sep. 30, 2019				5,488,261
Beginning balance at Sep. 30, 2019		$ (17,831)		$ 5	161	0	(17,997)
Convertible preferred stock, beginning balance (in shares) at Dec. 31, 2019		21,504,893
Convertible preferred stock, beginning balance at Dec. 31, 2019	[1]	$ 122,907
Convertible preferred stock, ending balance (in shares) at Mar. 31, 2020		21,504,893
Convertible preferred stock, ending balance at Mar. 31, 2020		$ 122,907
Common stock, beginning balance (in shares) at Dec. 31, 2019				5,660,391
Beginning balance at Dec. 31, 2019		(23,203)	[1]	$ 6	271	(18)	(23,462)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon vesting and exercise of options and vesting of restricted stock (in shares)				119,939
Issuance of common stock upon vesting and exercise of options and vesting of restricted stock		20			20
Stock-based compensation expense		188			188
Net unrealized gain (loss) on available-for-sale securities		158				158
Net loss		(6,995)					(6,995)
Common stock, ending balance (in shares) at Mar. 31, 2020				5,780,330
Beginning balance at Mar. 31, 2020		$ (29,832)		$ 6	479	140	(30,457)
Convertible preferred stock, beginning balance (in shares) at Dec. 31, 2019		21,504,893
Convertible preferred stock, beginning balance at Dec. 31, 2019	[1]	$ 122,907
Convertible preferred stock, ending balance (in shares) at Sep. 30, 2020		21,504,893
Convertible preferred stock, ending balance at Sep. 30, 2020		$ 122,907
Common stock, beginning balance (in shares) at Dec. 31, 2019				5,660,391
Beginning balance at Dec. 31, 2019		(23,203)	[1]	$ 6	271	(18)	(23,462)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net unrealized gain (loss) on available-for-sale securities		65
Net loss		(55,287)
Common stock, ending balance (in shares) at Sep. 30, 2020				6,284,326
Beginning balance at Sep. 30, 2020		$ (76,280)		$ 6	2,416	47	(78,749)
Convertible preferred stock, beginning balance (in shares) at Mar. 31, 2020		21,504,893
Convertible preferred stock, beginning balance at Mar. 31, 2020		$ 122,907
Convertible preferred stock, ending balance (in shares) at Jun. 30, 2020		21,504,893
Convertible preferred stock, ending balance at Jun. 30, 2020		$ 122,907
Common stock, beginning balance (in shares) at Mar. 31, 2020				5,780,330
Beginning balance at Mar. 31, 2020		(29,832)		$ 6	479	140	(30,457)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon vesting and exercise of options and vesting of restricted stock (in shares)				171,713
Issuance of common stock upon vesting and exercise of options and vesting of restricted stock		113			113
Stock-based compensation expense		293			293
Net unrealized gain (loss) on available-for-sale securities		24				24
Net loss		(8,579)					(8,579)
Common stock, ending balance (in shares) at Jun. 30, 2020				5,952,043
Beginning balance at Jun. 30, 2020		$ (37,981)		$ 6	885	164	(39,036)
Convertible preferred stock, ending balance (in shares) at Sep. 30, 2020		21,504,893
Convertible preferred stock, ending balance at Sep. 30, 2020		$ 122,907
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon vesting and exercise of options and vesting of restricted stock (in shares)				332,283
Issuance of common stock upon vesting and exercise of options and vesting of restricted stock		278			278
Stock-based compensation expense		1,253			1,253
Net unrealized gain (loss) on available-for-sale securities		(117)				(117)
Net loss		(39,713)					(39,713)
Common stock, ending balance (in shares) at Sep. 30, 2020				6,284,326
Beginning balance at Sep. 30, 2020		$ (76,280)		$ 6	$ 2,416	$ 47	$ (78,749)
[1]	The balance sheet as of December 31, 2019 is derived from the audited financial statements as of that date.

Condensed Statements of Conve_2

Condensed Statements of Convertible Preferred Stock and Stockholders' Deficit (Unaudited) (Parentheticals) $ in Thousands	3 Months Ended
	Sep. 30, 2019USD ($)$ / shares	Jun. 30, 2019USD ($)$ / shares
Convertible preferred stock, par value (in dollars per share) | $ / shares	$ 7.67	$ 7.67
Convertible Preferred Stock
Payments of stock issuance costs | $	$ 78	$ 70

Condensed Statements of Cash Fl

Condensed Statements of Cash Flows (unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities:
Net loss	$ (55,287)	$ (10,652)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	539	215
Net amortization/accretion on available-for-sale securities	105	0
Change in accrued interest on available-for-sale securities	71	0
Stock-based compensation expense	1,734	60
Noncash lease expense	1,994	183
Change in fair value of convertible notes payable	15,215	0
Debt issuance costs on convertible notes payable	3,889	0
Acquired in-process research and development	3,565	0
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(493)	(271)
Other long-term assets	150	(152)
Accounts payable	1,129	258
Accrued expenses	879	434
Right-of-use operating assets and liabilities, net	(588)	(183)
Other liabilities	4,816	(32)
Net cash used in operating activities	(22,282)	(10,140)
Cash flows from investing activities:
Purchase of property and equipment	(5,886)	(2,311)
Purchase of available-for-sale securities	(41,205)	0
Maturities of available-for-sale securities	45,611	0
Net cash used by investing activities	(1,480)	(2,311)
Cash flows from financing activities:
Principal payments on finance lease	(24)	(22)
Proceeds from issuance of common stock	411	56
Proceeds from issuance of preferred stock, net of financing fees	0	104,929
Proceeds from issuance of convertible notes, net of issuance costs	151,275	0
Payments of deferred offering costs	(2,248)	0
Net cash provided by financing activities	149,414	104,963
Net increase in cash and cash equivalents	125,652	92,512
Cash, cash equivalents and restricted cash at beginning of period	32,570	10,226
Cash, cash equivalents and restricted cash at end of period	158,222	102,738
Supplemental disclosures of non-cash activities:
Property and equipment additions included in accounts payable and accrued expenses	1,325	478
Right-of-use asset obtained in exchange for operating lease liability	30,898	0
Deferred offering costs included in accounts payable and accrued expenses	682	0
Preferred stock issuance costs in accounts payable and accrued expenses	$ 0	$ 8

Nature of Operations and Basis

Nature of Operations and Basis of Presentation	9 Months Ended
	Sep. 30, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Operations and Basis of Presentation	NATURE OF OPERATIONS AND BASIS OF PRESENTATION Kronos Bio, Inc. (Kronos or the Company), a Delaware corporation, was incorporated on June 2, 2017. The Company is a clinical-stage biopharmaceutical company dedicated to the discovery and development of novel cancer therapeutics designed to transform patient outcomes through a precision medicine strategy by targeting dysregulated transcription. The Company operates in one business segment, the development of biopharmaceutical products. Initial Public Offering In October 2020, the Company completed an initial public offering (IPO) of its common stock. In connection with its IPO, the Company issued and sold 15,131,579 shares of its common stock, which included 1,973,684 shares of its common stock issued and sold pursuant to the full exercise of the underwriters’ option to purchase additional shares, at a price to the public of $19.00 per share. As a result of the IPO, the Company received approximately $263.9 million in net proceeds, after deducting underwriting discounts and commissions of $20.1 million and offering expenses of approximately $3.5 million payable by the Company. At the closing of the IPO, 21,504,893 shares of outstanding convertible preferred stock were automatically converted to 22,687,625 shares of common stock and the 2020 Notes (See Note 15) were automatically converted into 9,610,713 shares of common stock. At the closing of the IPO, the Gilead Note (See Note 13) was also converted into 188,567 shares of common stock. Following the IPO, there were no shares of convertible preferred stock outstanding. As of September 30, 2020, the Company had incurred $2.9 million of deferred offering costs, which will be offset against the net proceeds received from the sale of common stock. Need for Additional Capital The Company has incurred net operating losses since its inception of $78.7 million as of September 30, 2020. The Company expects that its cash, cash equivalents and investments as of September 30, 2020, in addition to its approximate net cash proceeds of $263.9 million from the Company’s IPO, which closed on October 14, 2020, will enable it to fund its planned operating expenses and capital expenditure requirements for at least one year from the date of issuance of these condensed financial statements. Management expects to incur additional losses in the future to fund its operations and conduct product research and development and recognizes the need to raise additional capital to fully implement its business plan. The Company intends to raise additional capital through the issuance of equity securities, debt financings or other sources in order to further implement its business plan. However, if such financing is not available when needed and at adequate levels, the Company will need to reevaluate its operating plan and may be required to delay the development of its product candidates. The Company expects that its cash and cash equivalents and investments will be sufficient to fund its operations for a period of at least one year from the date the accompanying unaudited condensed consolidated financial statements are filed with the Securities and Exchange Commission (SEC). In March 2020, the World Health Organization declared the global novel coronavirus disease (COVID-19) outbreak a pandemic. The Company cannot at this time predict the specific extent, duration, or full impact that the COVID-19 pandemic will have on its financial condition and operations, including ongoing and planned clinical trials. The impact of the COVID-19 pandemic on the financial performance of the Company will depend on future developments, including the duration and spread of the pandemic and related governmental advisories and restrictions. These developments and the impact of the COVID-19 pandemic on the financial markets and the overall economy are highly uncertain. If the financial markets and/or the overall economy are impacted for an extended period, the Company’s results may be adversely affected. The future viability of the Company is dependent on its ability to generate cash from operating activities or to raise additional capital to finance its operations. The Company’s failure to raise capital as and when needed could have a material adverse effect on its financial condition and ability to pursue business strategies. The Company may not be able to obtain financing on acceptable terms, or at all, and the Company may not be able to enter into collaboration arrangements or obtain government grants. The terms of any financing may adversely affect the holdings or the rights of the Company’s stockholders. If the Company is unable to obtain funding, the Company could be forced to delay, reduce, or eliminate its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects. In the event that the Company requires additional funding, there can be no assurance that it will be successful in obtaining sufficient funding on terms acceptable to the Company to fund its continuing operations, if at all. Forward Stock Split On October 2, 2020, the Company filed an amendment to the Company’s amended and restated certificate of incorporation to effect a forward split of shares of the Company’s common stock on a 1.055-for-1 basis (the Forward Stock Split). In connection with the Forward Stock Split, the conversion ratio for the Company’s outstanding convertible preferred stock was proportionally adjusted such that the common stock issuable upon conversion of such preferred stock was increased in proportion to the Forward Stock Split. The par value of the common stock was not adjusted as the result of the Forward Stock Split. All references to common stock, options to purchase common stock, early exercised options, share data, per share data, convertible preferred stock (to the extent presented on an as-converted to common stock basis) and related information contained in these condensed financial statements have been retrospectively adjusted to reflect the effect of the Forward Stock Split for all periods presented.

Significant Accounting Policies

Significant Accounting Policies, Estimates and Judgments	9 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Significant Accounting Policies, Estimates and Judgments	SIGNIFICANT ACCOUNTING POLICIES, ESTIMATES AND JUDGMENTS 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 pursuant to Form 10-Q and Article 10 of Regulation S-X of the Securities and Exchange Commission (SEC). Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. Unaudited Interim Financial Information The accompanying balance sheet as of September 30, 2020, the statements of operations and comprehensive loss for the three and nine months ended September 30, 2020 and 2019, the statements of convertible preferred stock and stockholders’ deficit as of September 30, 2020 and 2019, the statements of cash flows for the nine months ended September 30, 2020 and 2019, and the financial data and other financial data and other information disclosed to in the notes to the condensed financial statements are unaudited. The unaudited interim financial statements have been prepared on a basis consistent with the Company’s audited annual financial statements and, in the opinion of management, reflect all adjustments, consisting solely of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of September 30, 2020 and the results of its operations the three and nine months ended September 30, 2020 and 2019. The results for the three and nine months ended September 30, 2020 are not necessarily indicative of results to be expected for the year ending December 31, 2020, any other interim periods, or any future year or period. These financial statements should be read in conjunction with the Company's audited financial statements included in the prospectus dated October 8, 2020 that forms a part of the Company's Registration Statements on Form S-1 (File No. 333-248925), as filed with the SEC pursuant to Rule 424 promulgated under the Securities Act of 1933, as amended, on October 9, 2020. 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, the disclosure of assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these condensed financial statements include, but are not limited to, the accrual of research and development expenses, the fair value of convertible notes and investments, income tax uncertainties, the valuation of equity instruments and the incremental borrowing rate for determining the operating lease assets and liabilities. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates. The full extent to which the COVID-19 pandemic will directly or indirectly impact the Company’s business, results of operations and financial condition, including expenses, clinical trials and research and development costs, will depend on future developments that are highly uncertain, including as a result of new information that may emerge concerning COVID-19 and the actions taken to contain or treat COVID-19, as well as the economic impact on local, regional, national and international markets. Research and Development Expenses Research and development (R&D) expenses consist primarily of personnel costs, including salaries, benefits and stock-based compensation, discovery research performed by contract research organizations (CROs), materials and supplies, licenses and fees and overhead allocations consisting of various support and facility-related costs. The Company expenses R&D costs as the services are performed or the goods are received. Research and development expenses for the three and nine months ended September 30, 2020 included $6.6 million related to acquired intangible assets as research and development costs pursuant to the Asset Purchase Agreement with Gilead (See Note 13) as, at the time of acquisition of the asset, the technology was under development; was not approved by the U.S. Food and Drug Administration or other regulatory agencies for marketing; had not reached technical feasibility; or otherwise had no foreseeable alternative future use. There were no acquired intangible assets expensed for the three and nine months ended September 30, 2019. CRO costs are a significant component of R&D expenses. The Company monitors levels of performance under each significant contract through communications with its CROs. The Company accrues costs for discovery research performed by CROs over the service periods specified in the contracts and adjusts our estimates, if required, based upon its ongoing review of the level of effort and costs actually incurred by the CROs. All of the Company’s material CRO contracts are terminable by the Company upon written notice and it is generally only liable for actual services completed by the CRO and certain non-cancellable expenses incurred at any point of termination. Cash and Cash Equivalents The Company considers all highly liquid investments with an original maturity of three months or less from the date of purchase to be cash equivalents. Restricted Cash The Company has deposited cash of $2.0 million as of September 30, 2020 to secure a letter of credit in connection with the lease of the 301 Binney facility (See Note 14). The Company has classified the restricted cash as a noncurrent asset on its balance sheets. Concentration of Credit Risk and Other Risks and Uncertainties Financial instruments, which potentially subject the Company to significant concentrations of credit risk, consist primarily of cash and cash equivalents and investments. The primary objectives for the Company’s investment portfolio are the preservation of capital and the maintenance of liquidity. The Company does not enter into any investment transaction for trading or speculative purposes. The Company’s investment policy limits investments to certain types of instruments such as certificates of deposit, money market instruments, obligations issued by the U.S. government and U.S. government agencies as well as corporate debt securities, and places restrictions on maturities and concentration by type and issuer. The Company maintains cash balances in excess of amounts insured by the FDIC and concentrated within a limited number of financial institutions. The accounts are monitored by management and management believes that the financial institutions are financially sound, and, accordingly, minimal credit risk exists with respect to these financial institutions. As of September 30, 2020 and December 31, 2019, the Company has not experienced any credit losses in such accounts or investments. The Company is subject to a number of risks common for biopharmaceutical companies, including, but not limited to, dependency on the clinical and commercial success of its product candidates, ability to obtain regulatory approval of its product candidates, the need for substantial additional financing to achieve its goals, uncertainty of broad adoption of its approved products, if any, by physicians and patients, significant competition, and untested manufacturing capabilities. Investments Investments are available-for-sale and are carried at estimated fair value. The Company’s valuations of available-for-sale securities are generally derived from independent pricing services based upon quoted prices in active markets for similar securities, with prices adjusted for yield and number of days to maturity, or based on industry models using data inputs, such as interest rates and prices that can be directly observed or corroborated in active markets. Management determines the appropriate classification of its investments in debt securities at the time of purchase. Investments with original maturities beyond three months at the date of purchase and which mature at, or less than 12 months from, the balance sheet date are classified as short-term investments. Unrealized gains and losses are excluded from earnings and are reported as components of comprehensive loss. The Company periodically evaluates whether declines in fair values of its available-for-sale 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 available-for-sale security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not that it will be required to sell any available-for-sale securities before recovery of its amortized cost basis. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on available-for-sale securities are included in interest income (expense), net. The cost of investments sold is based on the specific-identification method. Interest income on investments is included in interest income (expense), net on the Company’s statements of operations and comprehensive loss. Fair Value Measurement Assets and liabilities recorded at fair value on a recurring basis in the balance sheets are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements 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. 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. The fair value estimates are made at a discrete point in time based on relevant market information and information about the financial instruments. Fair value estimates are based on judgments regarding future expected loss experience, current economic conditions, risk characteristics of various financial instruments, and other factors. These estimates are subjective in nature and involve uncertainties and matters of significant judgment and, therefore, cannot be determined with precision. Changes in assumptions could significantly affect the estimates. As of September 30, 2020, the Company recorded both financial assets and financial liabilities requiring fair value measurement. The financial assets include cash equivalents and investments. The financial liabilities consist of the convertible promissory note (Gilead Note) from the Gilead Asset Purchase Agreement (See Note 13) and the convertible promissory notes (2020 Notes) sold in August 2020 (See Note 15). As of December 31, 2019, the Company recorded financial assets, including cash equivalents, and investments, requiring fair value measurement. There were no financial liabilities as of December 31, 2019. Fair Value Option Under U.S. GAAP, the Company has the irrevocable option to report most financial assets and financial liabilities at fair value on an instrument by instrument basis, with changes in fair value reported in the statements of operations. The option was elected for the treatment of the Company’s convertible promissory note from the Gilead Asset Purchase Agreement entered into in July 2020 (See Note 13) and the convertible promissory notes (2020 Notes) sold in August 2020 (See Note 15) in order to more accurately reflect the economic value of the notes. Property and Equipment, Net Property and equipment is stated at cost less accumulated depreciation and amortization. Depreciation and amortization are recognized using the straight-line method over the estimated useful lives of the respective assets . Leasehold improvements are amortized over the lesser of their useful lives or the remaining life of the lease. When assets are retired or otherwise disposed of, the cost and related accumulated depreciation and amortization are removed from the balance sheet and the resulting gain or loss is reflected in operations in the period realized . Repairs and maintenance costs are expensed as incurred. Estimated useful lives in years are generally as follows: Description Estimated Useful Life Lab equipment 3 to 7 years Leasehold improvements Shorter of useful life or lease term Deferred Offering Costs Offering costs, including legal, accounting, and filing fees related to the IPO, were deferred and were offset against the offering proceeds upon the completion of the IPO. As of September 30, 2020, $2.9 million of deferred offering costs have been capitalized, which are included in the accompanying balance sheet within other noncurrent assets. There were no deferred offering costs recorded as of December 31, 2019. Leases The Company determines if an arrangement is a lease at inception. Operating leases are included in operating lease right-of-use (ROU) assets, current portion of operating lease liabilities, and noncurrent operating lease liabilities on the Company’s balance sheet. Finance leases are included in property and equipment, current portion of other liabilities, and other noncurrent liabilities on the balance sheet. ROU assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent the Company’s obligation to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. As most of our leases do not provide an implicit rate, the Company uses its incremental borrowing rate based on the information available at commencement date in determining the present value of lease payments. The operating lease ROU asset also includes any lease payments and initial direct costs incurred, net of lease incentives. The Company’s lease terms may include options to extend or terminate the lease when it is reasonably certain that we will exercise that option. Lease expense for lease payments is recognized on a straight-line basis over the lease term. The Company elected to exclude from its balance sheets recognition of leases having a term of 12 months or less (short-term leases) and elected to not separate lease components and non-lease components for its long-term real-estate leases. Impairment of Long-Lived Assets Long-lived assets, including property and equipment, are reviewed for impairment whenever facts or circumstances either internally or externally may suggest that the carrying value of an asset or asset group may not be recoverable. Should there be an indication of impairment, the Company tests for recoverability by comparing the estimated undiscounted future cash flows expected to result from the use of the asset or asset group and its eventual disposition to the carrying amount of the asset or asset group. Any excess of the carrying value of the asset or asset group over its estimated fair value is recognized as an impairment loss. Stock-Based Compensation The Company measures stock-based awards granted to employees and nonemployees based on the fair value on the date of the grant and recognizes stock-based compensation expense of those awards over the requisite service period, which is generally the vesting period of the respective award. The Company calculates the fair value measurement of stock options using the Black-Scholes option-pricing model. Forfeitures are accounted for as they occur. As of September 30, 2020 and December 31, 2019, the Company has only issued stock options with service-based vesting conditions and records the expense for these awards using the straight-line method. Income Taxes The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the Company’s financial statements or tax returns. Under this method, deferred tax assets and liabilities are determined based on differences between the financial statement carrying amounts and the tax bases of the assets and liabilities using the enacted tax rates in effect in the years in which the differences are expected to reverse. A valuation allowance against deferred tax assets is recorded if, based on the weight of the available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. The Company accounts for uncertainty in income taxes recognized in the financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties. Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the three and nine months ended September 30, 2020, other comprehensive loss consisted of unrealized gains and losses from available-for-sale securities. There was no difference between net loss and comprehensive loss for the three and nine months ended September 30, 2019. Net Loss Per Share Basic net loss per share is computed using the weighted-average number of shares of common stock outstanding during the period. Diluted net loss per share is computed using the sum of the weighted-average number of shares of common stock outstanding during the period and the effect of dilutive securities. In periods in which the Company reports a net loss, diluted net loss per share is the same as basic net loss per share, since dilutive shares of common stock are not assumed to have been issued if their effect on net loss per share is anti-dilutive. Recently Adopted Accounting Pronouncements In August 2018, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2018-13, Fair Value Measurement (Topic 820) Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement (ASU 2018-13). The Company adopted ASU No. 2018-13 on January 1, 2019. This standard modifies certain disclosure requirements on fair value measurements. The adoption of this standard did not have a material impact on the Company’s financial statements. In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows, Restricted Cash (ASU 2016-18). This ASU requires changes in restricted cash during the period to be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. If cash, cash equivalents and restricted cash are presented in more than one line item on the balance sheet, the new guidance requires a reconciliation of the total in the statement of cash flows to the related captions in the balance sheet. The Company elected to adopt ASU 2016-18 on January 1, 2019 and has reflected the adoption in its financial statements. A reconciliation of the cash, cash equivalents, and restricted cash reported within the balance sheet that sum to the total of the same amounts shown in the statement of cash flows is as follows: September 30, 2020 2019 (in thousands) Cash and cash equivalents $ 156,196 $ 102,738 Restricted cash 2,026 — Total cash, cash equivalents and restricted cash as shown on the statements of cash flows $ 158,222 $ 102,738 In addition, the Company adopted ASU No 2016-15, Statement of Cash Flow (ASU 2016-15) in 2019. The guidance reduces diversity in how certain cash receipts and cash payments are presented and classified in the statements of cash flows. The adoption of ASU 2016-15 did not have a material impact on the Company’s condensed financial statements. In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses: Measurement of Credit Losses on Financial Instruments and also issued subsequent amendments to the initial guidance: ASU 2018-19, ASU 2019-04, ASU 2019-05, and ASU 2019-11. The standard requires measurement and recognition of expected credit losses for financial assets by requiring an allowance to be recorded as an offset to the amortized cost of such assets. For available-for-sale debt securities, expected credit losses should be estimated when the fair value of the debt securities is below their associated amortized costs. The standard will become effective for the Company in the first quarter of 2020, with early adoption permitted beginning the first quarter of 2019. The modified retrospective approach should be applied upon adoption of this new guidance. The Company’s financial instruments that are in the scope of ASU 2016-13 include available-for-sale debt securities. The Company adopted this standard on January 1, 2020 and this amendment did not have a material impact on its financial statements. In December 2019, the FASB issued ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes (ASU 2019-12), which simplifies the accounting for income taxes, eliminates certain exceptions within ASC 740, Income Taxes

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	FAIR VALUE MEASUREMENTS The Company follows authoritative accounting guidance, which among other things, defines fair value, establishes a consistent framework for measuring fair value and expands disclosure for each major asset and liability category measured at fair value on either a recurring or nonrecurring basis. Fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. The Company measures and reports its cash equivalents, investments, and convertible notes at fair value. Money market funds are measured at fair value on a recurring basis used quoted prices and are classified as Level 1. Investments are measured at fair value based on inputs other than quoted prices that are derived from observable market data and are classed as Level 2 inputs. Level 3 liabilities consist of the Company’s convertible notes payable. The determination of the fair values for the Gilead Note and the 2020 Notes and the change in the fair value of these notes are discussed in Note 13 and Note 15, respectively. Financial assets and liabilities subject to fair value measurements on a recurring basis and the level of inputs used in such measurements by major security type as of September 30, 2020 and December 31, 2019 were as follows: September 30, 2020 Level 1 Level 2 Level 3 Fair Value (in thousands) Financial Assets: Money market funds $ 84,635 $ — $ — $ 84,635 Certificates of deposit 1,234 — — 1,234 Corporate bonds — 18,021 — 18,021 U.S. treasury securities 40,926 — — 40,926 Total financial assets $ 126,795 $ 18,021 $ — $ 144,816 Financial Liabilities: 2020 Notes $ — $ — $ 168,506 $ 168,506 Gilead Note — — 5,488 5,488 Total financial liabilities $ — $ — $ 173,994 $ 173,994 December 31, 2019 Level 1 Level 2 Level 3 Fair Value (in thousands) Financial Assets: Money market funds $ 112 $ — $ — $ 112 Certificates of deposit 1,715 — — 1,715 Commercial paper — 4,489 — 4,489 Corporate bonds — 26,432 — 26,432 U.S. agency securities — 1,499 — 1,499 U.S. treasury securities 36,880 — — 36,880 Total financial assets $ 38,707 $ 32,420 $ — $ 71,127 The carrying amounts of accounts payable and accrued expenses approximate their fair values due to their short-term maturities. 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. Both observable and unobservable inputs were used to determine the fair value of positions that the Company has classified within the Level 3 category as of September 30, 2020. Recorded losses associated within the Level 3 category include changes to fair value and are recorded within change in fair value of convertible notes in the statement of operations. Refer to Note 13 and Note 15 for the fair value determination of the Level 3 financial liabilities. The Company did not have any financial assets or liabilities as of December 31, 2019 that required Level 3 inputs. There were no transfers of assets between the fair value measurement levels during any of the periods presented in the accompanying financial statements.

Investments

Investments	9 Months Ended
	Sep. 30, 2020
Investments, Debt and Equity Securities [Abstract]
Investments	INVESTMENTS The fair value and amortized cost of available-for-sale securities by major security type as of September 30, 2020 and December 31, 2019 were as follows: September 30, 2020 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market funds $ 84,635 $ — $ — $ 84,635 Certificates of deposit 1,225 9 — 1,234 Corporate bonds 18,011 18 (8) 18,021 U.S. treasury securities 40,898 30 (2) 40,926 Total cash equivalents and investments $ 144,769 $ 57 $ (10) $ 144,816 December 31, 2019 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market funds $ 112 $ — $ — $ 112 Certificates of deposit 1,715 — — 1,715 Commercial paper 4,490 — (1) 4,489 Corporate bonds 26,444 1 (13) 26,432 U.S. agency securities 1,500 — (1) 1,499 U.S. treasury securities 36,884 1 (5) 36,880 Total cash equivalents and investments $ 71,145 $ 2 $ (20) $ 71,127 These available-for-sale securities were classified on the Company’s balance sheets as of September 30, 2020 and December 31, 2019 as: Fair Value September 30, 2020 December 31, 2019 (in thousands) Cash equivalents $ 84,973 $ 6,751 Short-term investments 55,069 59,614 Long-term investments 4,774 4,762 Total cash equivalents and investments $ 144,816 $ 71,127 The fair values of available-for-sale securities by contractual maturity as of September 30, 2020 and December 31, 2019 were as follows: September 30, 2020 December 31, 2019 (in thousands) Due in 1 year or less $ 55,407 $ 66,253 Due in 1 to 2 years 4,774 4,762 Instruments not due at a single maturity date 84,635 112 Total cash equivalents and investments $ 144,816 $ 71,127

Prepaid Expenses And Other Curr

Prepaid Expenses And Other Current Assets	9 Months Ended
	Sep. 30, 2020
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid Expenses And Other Current Assets	PREPAID EXPENSES AND OTHER CURRENT ASSETS Prepaid expenses and other current assets consisted of the following as of September 30, 2020 and December 31, 2019: September 30, 2020 December 31, 2019 (in thousands) Accrued interest on short-term available-for-sale securities $ 217 $ 198 Prepaid equipment service contracts 432 191 Prepaid external research and development 410 113 Prepaid software 254 180 Prepaid insurance 36 23 Prepaid rent 3 196 Other prepaid expenses 276 218 Total prepaid expenses and other current assets $ 1,628 $ 1,119

Property and Equipment, Net

Property and Equipment, Net	9 Months Ended
	Sep. 30, 2020
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	PROPERTY AND EQUIPMENT, NET Property and equipment, net consisted of the following as of September 30, 2020 and December 31, 2019: September 30, 2020 December 31, 2019 (in thousands) Property and equipment: Lab equipment $ 5,256 $ 3,978 Finance lease on R&D equipment 139 139 Construction in progress 5,891 50 Total property and equipment 11,286 4,167 Less: Accumulated depreciation and amortization (985) (446) Total property and equipment, net $ 10,301 $ 3,721

Accrued Expenses and Current Po

Accrued Expenses and Current Portion of Other Liabilities	9 Months Ended
	Sep. 30, 2020
Payables and Accruals [Abstract]
Accrued Expenses and Current Portion of Other Liabilities	ACCRUED EXPENSES AND CURRENT PORTION OF OTHER LIABILITIES Accrued expenses consisted of the following as of September 30, 2020 and December 31, 2019: September 30, 2020 December 31, 2019 (in thousands) Accrued compensation $ 1,107 $ 528 Accrued franchise tax 38 43 External research and development 469 241 Accrued offering costs 83 — Other accrued expenses 83 6 Total accrued expenses $ 1,780 $ 818 Current portion of other liabilities consist of the following as of September 30, 2020 and December 31, 2019: September 30, 2020 December 31, 2019 (in thousands) Current portion of finance lease liability $ 14 $ 32 Current portion of unvested early exercised share liability 2,123 56 Total current portion of other current liabilities $ 2,137 $ 88

Preferred Stock

Preferred Stock	9 Months Ended
	Sep. 30, 2020
Temporary Equity Disclosure [Abstract]
Preferred Stock	PREFERRED STOCK On May 22, 2018, the Company completed a private placement (Series Seed Financing) in which it issued an aggregate of 7,806,977 shares of its Series Seed Convertible Preferred Stock (Series Seed Preferred Stock) at a purchase price of $2.30769 per share, for aggregate gross proceeds of $18.0 million. On July 1, 2019, the Company completed a private placement (the Series A Financing) in which it issued 13,697,916 shares of its Series A Convertible Preferred Stock (Series A Preferred Stock) at a purchase price of $7.6654 per share, for aggregate gross proceeds of $105.0 million less issuance costs of $0.1 million. The Preferred Stock (as defined below) consisted of the following as of September 30, 2020 and December 31, 2019: September 30, 2020 Preferred Stock Authorized Preferred Carrying Liquidation Common (in thousands, except share amounts) Series Seed Preferred Stock 7,806,977 7,806,977 $ 17,985 $ 18,016 8,236,347 Series A Preferred Stock 13,700,000 13,697,916 104,922 105,000 14,451,278 Total 21,506,977 21,504,893 $ 122,907 $ 123,016 22,687,625 December 31, 2019 Preferred Stock Authorized Preferred Carrying Liquidation Common (in thousands, except share amounts) Series Seed Preferred Stock 7,806,977 7,806,977 $ 17,985 $ 18,016 8,236,347 Series A Preferred Stock 13,700,000 13,697,916 104,922 105,000 14,451,278 Total 21,506,977 21,504,893 $ 122,907 $ 123,016 22,687,625 Upon the completion of the IPO (See Note 1), all outstanding shares of convertible preferred stock were automatically converted into 22,687,625 shares of common stock.

Common Stock

Common Stock	9 Months Ended
	Sep. 30, 2020
Equity [Abstract]
Common Stock	COMMON STOCKPursuant to the Company’s amended and restated certificate of incorporation, filed on July 1, 2019, the Company was authorized to issue up to 40,000,000 shares of its common stock, par value $0.001. As of September 30, 2020 and December 31, 2019, there were 6,284,326 and 5,660,391 shares issued and outstanding, respectively.Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company's stockholders. Subject to the rights of the Preferred Stock, holders of the Company’s common stock are entitled to receive dividends, as may be declared by the Board of Directors.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2020
Share-based Payment Arrangement [Abstract]
Stock-Based Compensation	STOCK-BASED COMPENSATION 2017 Equity Incentive Plan The Company's 2017 Equity Incentive Plan, as amended (the 2017 Plan), provides that the Company may sell or issue shares of common stock or restricted common stock, or to grant incentive stock options or nonqualified stock options for the purchase of shares of common stock, to employees, members of the Board of Directors, and consultants of the Company. The exercise prices, vesting, and other restrictions are determined at the discretion of the Board of Directors, or a designated committee thereof, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the common share on the date of grant and the term of stock option may not be greater than 10 years. The Company recognizes the impact of forfeitures on stock-based compensation expense as forfeitures occur. The Company applies the straight-line method of expense recognition to all awards with only service-based vesting conditions. Vesting periods are determined at the discretion of the Board of Directors. Stock options typically vest over four years. The maximum contractual term is 10 years. As of September 30, 2020 and December 31, 2019, t he total number of shares of the Company’s common stock that may have been issued was 6,857,500 and 3,692,500 shares, respectively. As of September 30, 2020 and December 31, 2019, there were 358,596 and 1,365,647 shares, respectively, reserved by the Company under the 2017 Plan for the future issuance of equity awards. Stock Option Valuation The Company estimates the fair value of each stock option grant on the date of grant using the Black-Scholes option-pricing model. This model requires the use of highly subjective assumptions to determine the fair value of stock-based awards, including: • Fair Value of Common Stock —In order to determine the fair value of the Company’s common stock underlying option grants, the Board of Directors considered, among other things, valuations of the Company’s common stock prepared by an unrelated third-party valuation firm in accordance with the guidance provided by the American Institute of Certified Public Accountants Practice Guide, Valuation of Privately-Held-Company Equity Securities Issued as Compensation. Other objective and subjective factors considered included the Company’s stage of development and material risks related to its business, the progress of its research and development programs, its business conditions and projections, its financial position and its historical and forecasted performance and operating results, the lack of an active public market for its securities, its Preferred Stock, biopharmaceutical company performance, the likelihood of achieving a liquidity event, the hiring of key personnel and the experience of management, industry trends and developments, and external market conditions and industry trends. • Expected Term —The expected term represents the period that the stock-based awards are expected to be outstanding. The Company uses the simplified method to determine the expected term, which is based on the average of the time-to-vesting and the contractual life of the options. • Expected Volatility —Since the Company is not yet a public company and does not have any trading history for its common stock, the expected volatility is estimated based on the average historical volatilities of common stock of comparable publicly traded entities over a period equal to the expected term of the stock option grants. The comparable companies are chosen based on their size, stage in the product development cycle or area of specialty. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of our own stock price becomes available. • Risk-Free Interest Rate —The risk-free interest rate is based on the U.S. Treasury yield in effect at the time of grant for zero-coupon U.S. Treasury notes with maturities approximately equal to the expected term of the awards. • Expected Dividend —The Company has never paid dividends on its common stock and has no plans to pay dividends on its common stock. Therefore, the Company used an expected dividend yield of zero. The Black-Scholes option-pricing model assumptions that the Company used to determine the grant-date fair value of stock options for the three and nine months ended September 30, 2020 and 2019 were as follows, presented on a weighted-average basis: Three months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 Fair value of common stock per share $ 6.79 $ 2.53 $ 4.70 $ 1.35 Expected term (in years) 5.99 6.03 5.99 6.06 Expected volatility 81.72% 66.88% 76.93% 76.97% Risk-free interest rate 0.36% 1.67% 0.73% 2.15% Expected dividend —% —% —% —% The weighted-average grant-date fair value per share of stock options granted, using the assumptions listed above, was $4.66 and $1.54 during the three months ended September 30, 2020 and 2019, respectively, and $3.16 and $0.88 during the nine months ended September 30, 2020 and 2019, respectively. The weighted-average grant-date fair value per share of stock options vested was $0.69 and $0.09 during the nine months ended September 30, 2020 and 2019, respectively. Stock Options Stock option activity under the 2017 Plan as of September 30, 2020 is summarized as follows: Number of Options Weighted-Average Exercise Price Weighted-Average Remaining Contractual Term Aggregate Intrinsic Value (in years) (in thousands) Balance at December 31, 2019 1,726,233 $ 1.19 9.05 $ 27,287 Granted 4,015,631 4.70 Forfeited (117,921) 0.76 Exercised (406,057) 1.01 Balance at September 30, 2020 5,217,886 $ 3.91 9.34 $ 68,291 Exercisable at September 30, 2020 69,910 $ 1.33 8.43 $ 1,096 Unvested and expected to vest at September 30, 2020 5,147,976 $ 3.95 9.36 $ 67,195 The aggregate intrinsic values of options outstanding, exercisable, 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, as determined by the Board of Directors, as of the end of the respective period. The intrinsic value of options exercised for the three months ended September 30, 2020 and 2019 was $1.5 million a nd $0.2 million, respectively, and $2.0 million and $0.4 million for the nine months ended September 30, 2020 and 2019 respectively, determined as of the applicable date of exercise. There was no future tax benefit related to options exercised, as the Company had accumulated net operating losses as of September 30, 2020 and December 31, 2019. Stock-Based Compensation Stock-based compensation expense was classified in the Company’s statements of operations and comprehensive loss for the three and nine months ended September 30, 2020 and 2019 as follows: Three months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 (in thousands) Research and development expenses $ 246 $ 9 $ 541 $ 20 General and administrative expenses 1,007 14 1,193 40 Total stock-based compensation expense $ 1,253 $ 23 $ 1,734 $ 60 As of September 30, 2020, total unrecognized compensation cost related to the unvested share-based awards was $14.1 million, which is expected to be recognized over a weighted average period of 3.53 years. Early Exercised Options The Company allows certain of its employees and its consultants to exercise options granted under the 2017 Plan prior to vesting. The shares related to early exercised stock options are subject to the Company’s lapsing repurchase right upon termination of employment or service on the Board of Directors at the lesser of the original purchase price or fair market value at the time of repurchase. In order to vest, the holders are required to provide continued service to the Company. The early exercise by an employee or consultant of a stock option is not considered to be a substantive exercise for accounting purposes, and therefore the payment received by the employer for the exercise price is recognized as a liability. For accounting purposes, unvested early exercised shares are not considered issued and outstanding and therefore not reflected as issued and outstanding in the accompanying statements of convertible preferred stock and stockholders' deficit until the awards vest. The deposits received are initially recorded in current portion of other liabilities and other noncurrent liabilities for the noncurrent portion. The liabilities are reclassified to common stock and paid-in capital as the repurchase right lapses. During the three months ended September 30, 2020 and 2019, and nine months ended September 30, 2020 and 2019, 850,937, 60,924, 1,551,361, and 60,924 shares of the Company’s common stock were early exercised, respectively. At September 30, 2020 and December 31, 2019, there was $2.1 million and $0.1 million recorded in current portion of other liabilities, and $2.8 million and $0.1 million recorded in other noncurrent liabilities, respectively, related to shares held by employees and nonemployees that were subject to repurchase. Restricted Stock

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Income Taxes	INCOME TAXES The Company did not recorded any income tax expense for the three and the nine months ended September 30, 2020 and 2019. The Company has incurred net operating losses for all the periods presented and has not reflected any benefit of such net operating loss carryforwards in the accompanying financial statements. The Company has recorded a full valuation allowance against all of its deferred tax assets as it is not more likely than not that such assets will be realized in the near future. It is the Company’s policy to record penalties and interest related to income taxes as a component of income tax expense. The Company has not recorded any interest or penalties related to income taxes during the three and nine months ended September 30, 2020 and 2019. The Company has not identified any new uncertain tax positions as of September 30, 2020. Unrecognized tax benefits are not expected to change during the next 12 months. The reversal of the unrecognized tax benefits would not affect the effective tax rate. The Company is subject to examination by U.S. federal and state tax authorities for all years since its inception.

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2020
Earnings Per Share [Abstract]
Net Loss Per Share	NET LOSS PER SHARE The following table summarizes the computation of basic and diluted net loss per share of the Company for the three and nine months ended September 30, 2020 and 2019: Three months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 (in thousands, except share and per share amounts) Net loss $ (39,713) $ (4,013) $ (55,287) $ (10,652) Weighted-average common stock outstanding, basic and diluted 6,127,146 5,409,433 5,886,191 5,196,681 Net loss per share, basic and diluted $ (6.48) $ (0.74) $ (9.39) $ (2.05) The Company’s potentially dilutive securities, which include the Preferred Stock and options to purchase shares of the Company's common stock, have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted-average number of shares of common stock outstanding used to calculate both basic and diluted net loss per share is the same. The Company excluded the following potential shares of common stock, presented based on amounts outstanding at each stated period end, from the computation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect: September 30, 2020 September 30, 2019 Convertible preferred stock 22,687,625 22,687,625 2020 Notes 10,741,406 — Gilead Note 210,752 — Stock options to purchase common stock 3,359,622 389,086 Early exercised stock options subject to future vesting 1,858,264 854,123 Restricted stock award subject to future vesting 271,683 242,123 Total 39,129,352 24,172,957

Commitment and Contingencies

Commitment and Contingencies	9 Months Ended
	Sep. 30, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	COMMITMENTS AND CONTINGENCIES Asset Purchase Agreement In July 2020, the Company entered into an asset purchase agreement (Gilead Asset Purchase Agreement) with Gilead Sciences, Inc. (Gilead), pursuant to which the Company acquired certain assets from Gilead related to entospletinib (ENTO) and lanraplenib (LANRA), and patents and other intellectual property covering or related to the development, manufacture and commercialization of ENTO and LANRA. In consideration for such assets, on the date of the Gilead Asset Purchase Agreement, the Company made a $3.0 million upfront cash payment and issued a $3.0 million principal amount convertible promissory note to Gilead (Gilead Note)The Company also made a $0.7 million payment to reimburse Gilead for certain liabilities assumed by the Company pursuant to the Gilead Asset Purchase Agreement. In addition, the Company is required to make milestone payments upon successful achievement of certain regulatory and sales milestones for ENTO, LANRA and other selective spleen tyrosine kinase inhibitor compounds covered by the patent rights acquired pursuant to the Gilead Asset Purchase Agreement and developed by the Company as a back-up to ENTO or LANRA (Other Compounds). Upon initiation of the Company’s planned registrational Phase 2/3 clinical trial of ENTO in combination with induction chemotherapy in acute myeloid leukemia patients with NPM1 mutations, the Company will be required to pay a milestone to Gilead of $29.0 million, and upon successful completion of certain other regulatory milestones in the United States, European Union and United Kingdom for ENTO, LANRA and any Other Compounds, across up to two distinct indications, the Company will be required to pay to Gilead an aggregate total of $51.3 million. Upon achieving certain thresholds for the aggregate annual net sales of ENTO, LANRA and any Other Compounds combined, the Company would owe to Gilead potential milestone payments totaling $115.0 million. Gilead is also eligible to receive (i) tiered marginal royalties ranging from the very low-teens to high-teens on annual worldwide net sales of ENTO, (ii) tiered marginal royalties ranging from high-single digits to the mid-teens on annual worldwide net sales of LANRA and (iii) tiered marginal royalties ranging from low single digits to mid-single digits on annual worldwide net sales of any Other Compounds. The royalties in the foregoing clauses are subject to reduction, on a country-by-country basis, for products not covered by certain claims within the assigned patents, for generic entry and, in the case of ENTO and LANRA, for any royalties paid for future licenses of third party intellectual property required to develop or commercialize ENTO or LANRA. The Company’s royalty obligation with respect to a given product in a given country begins upon the first commercial sale of such product in such country and ends on the latest of (i) expiration of the last claim of a defined set of the assigned patent rights covering such product in such country; (ii) loss of exclusive data or marketing rights to such product in such country; or (iii) 10 years from the first commercial sale of such product in such country. Under the Gilead Asset Purchase Agreement, the Company is required to use commercially reasonable efforts to develop, obtain regulatory approval for and commercialize either ENTO or LANRA. The Gilead Note accrues interest at a rate of 6% per year, compounded annually, and has a maturity date of July 14, 2020. The Gilead Note provides that, upon the completion of the Company’s IPO, the Gilead Note will settle through the Company payment to Gilead of $6.0 million plus unpaid accrued interest unless Gilead elected, within a specified time, that the Gilead Note, including unpaid accrued interest, be converted into shares of the Company’s common stock. On issuance, the Company elected to account for the Gilead Note at fair value with any changes in fair value being recognized through the statements of operations until the Gilead Note settled. The fair value of the Gilead Note was determined to be $3.6 million on issuance and $5.5 million as of September 30, 2020. For the three and nine months ended September 30, 2020, the Company recognized $1.9 million in the accompanying statements of operations and comprehensive loss as the change in fair value of the 2020 Gilead Note. The Company determined the fair value of the Gilead Note as of September 30, 2020 by determining the present value of the Gilead Note if they were to settle either upon closing of an IPO, a deemed liquidation, or a qualified or non-qualified equity fundraise. Key valuation assumptions are as follows: As of September 30, 2020 Fundraise scenario IPO cash payout scenario IPO conversion scenario Liquidation cash payout scenario Liquidation conversion scenario Remaining term (years) 1.78 0.08 0.08 1.79 1.79 Discount rate 24.2 % 24.2 % 24.2 % 24.2 % 24.2 % Weighting 1.5 % 95.0 % — % 2.0 % 1.5 % The Company determined the fair value of the Gilead Note at inception by determining the present value of the Gilead Note if they were to settle either upon closing of an IPO, a deemed liquidation, or a qualified or non-qualified equity fundraise. Key valuation assumptions are as follows: At inception Fundraise scenario IPO cash payout scenario IPO conversion scenario Liquidation cash payout scenario Liquidation conversion scenario Remaining term (years) 2.00 0.30 0.30 2.00 2.00 Discount rate 24.2 % 24.2 % 24.2 % 24.2 % 24.2 % Weighting 18.6 % 12.9 % — % 50.0 % 18.6 % Purchase Commitments In the normal course of business, the Company enters into contracts with CROs for preclinical studies and other vendors for other services and products for operating purposes. These agreements generally provide for termination or cancellation, other than for costs already incurred. Contingencies In the normal course of business, the Company enters into contracts and agreements that contain a variety of representations and warranties and provide for general indemnifications. The Company’s exposure under these agreements is unknown, because it involves claims that may be made against the Company in the future, but have not yet been made. The Company accrues a liability for such matters when it is probable that future expenditures will be made and such expenditures can be reasonably estimated. Indemnification In accordance with the Company’s amended and restated certificate of incorporation and amended and restated bylaws, the Company has indemnification obligations to its officers and directors for certain events or occurrences, subject to certain limits, while they are serving in such capacity. There have been no claims to date, and the Company has a directors and officers liability insurance policy that may enable it to recover a portion of any amounts paid for future claims.

Leases

Leases	9 Months Ended
	Sep. 30, 2020
Leases [Abstract]
Leases	LEASES In May 2020, the Company amended its month-to-month rental agreement for the 21 Erie Street, Cambridge, Massachusetts lab space, to extend its non-cancellable lease term. The amendment was executed on May 5, 2020 and has a term expiration date of May 31, 2021. As the lease term is longer than 12 months, the Company determined that this amendment requires assessment under ASC 842. In connection with the lease, the Company recognized an operating lease right-of-use asset of $1.6 million and an aggregate lease liability of $1.0 million on the September 30, 2020 balance sheet. The remaining lease term is 8 months, and the estimated incremental borrowing rate is 12.71%. In March 2020, the Company entered into an 11-year lease agreement to move its research and development operations from 21 Erie Street, Cambridge, Massachusetts, to a 40,514 square-feet facility at 301 Binney Street, Cambridge, Massachusetts. The new lease commenced on March 1, 2020 with a monthly base rent of $0.3 million. The initial rent payment was paid as of September 30, 2020, with rent payments escalating 3.0% annually after the initial 12 payments. As discussed in Note 2, the Company executed a letter of credit for $2.0 million in connection with the lease. The lease includes $3.7 million in certain tenant improvement allowances, which the Company included in its calculation of the right-of-use asset in the lease at commencement. In connection with the lease, the Company recognized an operating lease right-of-use asset of $25.7 million and an aggregate lease liability of $27.7 million on the September 30, 2020 balance sheet. The remaining lease term is 10 years and 5 months, and the estimated incremental borrowing rate is 8.50% . In July 2018, the Company entered into a lease agreement for a 4,661 square-foot office space to be used for general and administrative activities in San Mateo, California. The lease commenced on August 1, 2018 and has a 37-month initial term expiring on August 31, 2021. The lease also contains an option for the Company to extend the lease upon its initial expiration. In connection with the lease, the Company made a one-time cash security deposit in the amount of $27.8 thousand. In May 2020, the Company amended its agreement to extend the lease for its office in San Mateo, California through April 2025. The initial annual base rent is $26.1 thousand, and such amount will increase by 3% annually on each anniversary of the commencement date. In connection with the lease, the Company recognized an operating lease right-of-use asset of $1.2 million $0.5 million and an aggregate lease liability of $1.2 million and $0.5 million on the September 30, 2020 and December 31, 2019 balance sheets, respectively. The Company expanded to an adjacent suite in July 2020, which is treated as a separate lease for accounting purposes. The additional space is approximately 3,414 square feet and will similarly be used for general and administrative activities. The lease commenced on July 1, 2020 and will expire on April 30, 2025. In connection with the additional space leased, the Company made a one-time additional cash security deposit in the amount of $24.8 thousand. The initial base rent for the expansion is $19.2 thousand, and such amount will increase by 3% annually on each anniversary of the commencement date. In connection with the lease, the Company recognized an operating lease right-of-use asset of $0.9 million and an aggregate lease liability of $0.9 million on the September 30, 2020 balance sheet. The remaining lease term is for both spaces is 4 years and 7 months, and the estimated incremental borrowing rate is 12.07%. In March 2018, the Company entered into a finance lease for R&D equipment that has a bargain purchase option at the end of its three-year term. The finance lease is included in property and equipment, net, other current liabilities, and other noncurrent liabilities on the Company’s balance sheets. The following table summarizes the presentation of the Company’s finance lease in its balance sheets as of September 30, 2020 and December 31, 2019: Balance Sheet Caption September 30, 2020 December 31, 2019 (in thousands) Assets: Property and equipment, net $ 88 $ 103 Liabilities: Current portion of other liabilities 14 32 Other noncurrent liabilities — 5 Total finance lease liabilities $ 14 $ 37 The following table summarizes the presentation of the Company’s operating leases in its balance sheets as of September 30, 2020 and December 31, 2019: Balance Sheet Caption September 30, 2020 December 31, 2019 (in thousands) Assets: Operating lease assets $ 29,355 $ 473 Liabilities: Current portion of operating lease liabilities $ 1,300 $ 285 Noncurrent operating lease liabilities 29,484 211 Total operating lease liabilities $ 30,784 $ 496 The following table summarizes the effect of finance lease costs in the Company’s statements of operations and comprehensive loss for the three and nine months ended September 30, 2020 and 2019: Statement of Operations and Comprehensive Loss Caption Three months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 (in thousands) Research and development $ 5 $ 5 $ 15 $ 15 Interest income (expense), net — 1 1 3 Total finance lease cost $ 5 $ 6 $ 16 $ 18 The following table summarizes the effect of operating lease costs in the Company’s statements of operations and comprehensive loss for the three and nine months ended September 30, 2020 and 2019 : Statement of Operations and Comprehensive Loss Caption Three months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 (in thousands) Research and development $ 1,397 $ — $ 2,843 $ — General and administrative 388 78 890 233 Total operating lease cost $ 1,785 $ 78 $ 3,733 $ 233 The Company made cash payments of $0.7 million, $0.1 million, $1.3 million, and $0.2 million under the lease agreements during the three months ended September 30, 2020 and 2019 and the nine months ended September 30, 2020 and 2019, respectively. The undiscounted future non-cancellable lease payments under the Company's operating leases as of September 30, 2020 for the next five years and thereafter is expected to be as follows: Period Ending December 31, Amount (in thousands) Remaining three months of 2020 $ 1,329 2021 5,770 2022 4,789 2023 4,933 2024 and thereafter 35,915 Total undiscounted lease payments 52,736 Less: Present value adjustment (18,238) Less: Tenant improvement allowance (3,714) Present value of operating lease liabilities $ 30,784
Leases	LEASES In May 2020, the Company amended its month-to-month rental agreement for the 21 Erie Street, Cambridge, Massachusetts lab space, to extend its non-cancellable lease term. The amendment was executed on May 5, 2020 and has a term expiration date of May 31, 2021. As the lease term is longer than 12 months, the Company determined that this amendment requires assessment under ASC 842. In connection with the lease, the Company recognized an operating lease right-of-use asset of $1.6 million and an aggregate lease liability of $1.0 million on the September 30, 2020 balance sheet. The remaining lease term is 8 months, and the estimated incremental borrowing rate is 12.71%. In March 2020, the Company entered into an 11-year lease agreement to move its research and development operations from 21 Erie Street, Cambridge, Massachusetts, to a 40,514 square-feet facility at 301 Binney Street, Cambridge, Massachusetts. The new lease commenced on March 1, 2020 with a monthly base rent of $0.3 million. The initial rent payment was paid as of September 30, 2020, with rent payments escalating 3.0% annually after the initial 12 payments. As discussed in Note 2, the Company executed a letter of credit for $2.0 million in connection with the lease. The lease includes $3.7 million in certain tenant improvement allowances, which the Company included in its calculation of the right-of-use asset in the lease at commencement. In connection with the lease, the Company recognized an operating lease right-of-use asset of $25.7 million and an aggregate lease liability of $27.7 million on the September 30, 2020 balance sheet. The remaining lease term is 10 years and 5 months, and the estimated incremental borrowing rate is 8.50% . In July 2018, the Company entered into a lease agreement for a 4,661 square-foot office space to be used for general and administrative activities in San Mateo, California. The lease commenced on August 1, 2018 and has a 37-month initial term expiring on August 31, 2021. The lease also contains an option for the Company to extend the lease upon its initial expiration. In connection with the lease, the Company made a one-time cash security deposit in the amount of $27.8 thousand. In May 2020, the Company amended its agreement to extend the lease for its office in San Mateo, California through April 2025. The initial annual base rent is $26.1 thousand, and such amount will increase by 3% annually on each anniversary of the commencement date. In connection with the lease, the Company recognized an operating lease right-of-use asset of $1.2 million $0.5 million and an aggregate lease liability of $1.2 million and $0.5 million on the September 30, 2020 and December 31, 2019 balance sheets, respectively. The Company expanded to an adjacent suite in July 2020, which is treated as a separate lease for accounting purposes. The additional space is approximately 3,414 square feet and will similarly be used for general and administrative activities. The lease commenced on July 1, 2020 and will expire on April 30, 2025. In connection with the additional space leased, the Company made a one-time additional cash security deposit in the amount of $24.8 thousand. The initial base rent for the expansion is $19.2 thousand, and such amount will increase by 3% annually on each anniversary of the commencement date. In connection with the lease, the Company recognized an operating lease right-of-use asset of $0.9 million and an aggregate lease liability of $0.9 million on the September 30, 2020 balance sheet. The remaining lease term is for both spaces is 4 years and 7 months, and the estimated incremental borrowing rate is 12.07%. In March 2018, the Company entered into a finance lease for R&D equipment that has a bargain purchase option at the end of its three-year term. The finance lease is included in property and equipment, net, other current liabilities, and other noncurrent liabilities on the Company’s balance sheets. The following table summarizes the presentation of the Company’s finance lease in its balance sheets as of September 30, 2020 and December 31, 2019: Balance Sheet Caption September 30, 2020 December 31, 2019 (in thousands) Assets: Property and equipment, net $ 88 $ 103 Liabilities: Current portion of other liabilities 14 32 Other noncurrent liabilities — 5 Total finance lease liabilities $ 14 $ 37 The following table summarizes the presentation of the Company’s operating leases in its balance sheets as of September 30, 2020 and December 31, 2019: Balance Sheet Caption September 30, 2020 December 31, 2019 (in thousands) Assets: Operating lease assets $ 29,355 $ 473 Liabilities: Current portion of operating lease liabilities $ 1,300 $ 285 Noncurrent operating lease liabilities 29,484 211 Total operating lease liabilities $ 30,784 $ 496 The following table summarizes the effect of finance lease costs in the Company’s statements of operations and comprehensive loss for the three and nine months ended September 30, 2020 and 2019: Statement of Operations and Comprehensive Loss Caption Three months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 (in thousands) Research and development $ 5 $ 5 $ 15 $ 15 Interest income (expense), net — 1 1 3 Total finance lease cost $ 5 $ 6 $ 16 $ 18 The following table summarizes the effect of operating lease costs in the Company’s statements of operations and comprehensive loss for the three and nine months ended September 30, 2020 and 2019 : Statement of Operations and Comprehensive Loss Caption Three months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 (in thousands) Research and development $ 1,397 $ — $ 2,843 $ — General and administrative 388 78 890 233 Total operating lease cost $ 1,785 $ 78 $ 3,733 $ 233 The Company made cash payments of $0.7 million, $0.1 million, $1.3 million, and $0.2 million under the lease agreements during the three months ended September 30, 2020 and 2019 and the nine months ended September 30, 2020 and 2019, respectively. The undiscounted future non-cancellable lease payments under the Company's operating leases as of September 30, 2020 for the next five years and thereafter is expected to be as follows: Period Ending December 31, Amount (in thousands) Remaining three months of 2020 $ 1,329 2021 5,770 2022 4,789 2023 4,933 2024 and thereafter 35,915 Total undiscounted lease payments 52,736 Less: Present value adjustment (18,238) Less: Tenant improvement allowance (3,714) Present value of operating lease liabilities $ 30,784

2020 Notes

2020 Notes	9 Months Ended
	Sep. 30, 2020
Debt Disclosure [Abstract]
2020 Notes	2020 Notes In August 2020, the Company entered into a note purchase agreement pursuant to which it sold and issued and aggregate of $155.2 million in convertible promissory notes (2020 Notes) and received net cash proceeds of $151.3 million. The 2020 Notes do not accrue interest. The 2020 Notes were settled in exchange for 9,610,713 shares of common stock in connection with the closing of the Company’s IPO (See Note 1) at a settlement per share price equal to 85% of the IPO price per share. The 2020 Notes also contained other settlement provisions if an IPO did not occur within 18-months of the 2020 Notes issuance date. If the Company was acquired, completed a business combination resulting in a change of control or sold all or substantially all of its assets (each, a liquidation transaction) prior to the 18-month anniversary of the issuance dates of the 2020 Notes, the 2020 Notes unless previously settled into shares of common stock in the IPO, will settle into shares of the Company’s common stock at a price per share equal to 85% of the estimated fair value of the consideration per share payable to the holders of its common stock in connection with such liquidation transaction. If neither the IPO nor a liquidation transaction occurs prior to the 18th-month anniversary of the issuance dates of the 2020 Notes, the 2020 Notes will be converted into shares of newly designated Series B convertible preferred stock of the Company at a settlement price that will be determined based on a stipulation $500.0 million valuation of the Company and its fully diluted capitalization as of immediately prior to the conversion of the 2020 Notes. The 2020 Notes contain additional redemption features contingent upon the occurrence of certain future events. On issuance, the Company elected to account for the 2020 Notes at fair value with any changes in fair value being recognized through the statements of operations until the 2020 Notes settled. On issuance, the fair value of the 2020 Notes was determined to be equal to $155.2 million, which is the principal amount of the 2020 Notes, and total debt issuance costs of $3.9 million were expensed and recognized as interest expense in the accompanying statements of operations and comprehensive loss. An additional $50 thousand was reimbursed to investors for legal costs incurred and netted against total proceeds received on the Company’s balance sheet. Fair Value The Company determined the fair value of the 2020 Notes as of September 30, 2020 by determining the present value of the 2020 Notes if they were to settle either upon closing of an IPO, in shares of Series B convertible preferred stock 18-months after issuance (February 2022), or upon a financing scenario. Key valuation assumptions are as follows: As of September 30, 2020 IPO Scenario Series B Scenario Financing Scenario Remaining term (years) 0.08 1.39 1.39 Discount rate 24.2 % 24.2 % 24.2 % Weighting 95.0 % 2.5 % 2.5 % The fair value of the 2020 Notes was determined to be $155.2 million on issuance and $168.5 million as of September 30, 2020. For the three and nine months ended September 30, 2020, the Company recognized $13.3 million in the accompanying statements of operations and comprehensive loss as the change in fair value of the 2020 Notes.

Related Parties

Related Parties	9 Months Ended
	Sep. 30, 2020
Related Party Transactions [Abstract]
Related Parties	RELATED PARTIES On December 1, 2017, the Company entered into a three-year services agreement with Two River Consulting, LLC (Two River) to provide various clinical development, operational, managerial, accounting and financial, and administrative services to the Company. Arie Belldegrun, M.D., FACS, the Chairman of the Board of Directors, is the Chairman of Two River. Mr. Joshua Kazam and Mr. David Tanen, each a director of the Company, are each partners of Two River. Mr. Tanen additionally serves as our Corporate Secretary. Mr. Christopher Wilfong, a strategic advisor to the Company, is an Operating Partner of Two River and Mr. Sean Algeo, serving as the Company’s Treasurer, is the Chief Financial Officer of Two River. During the three months ended September 30, 2020 and 2019 and the nine months ended September 30, 2020 and 2019, the Company incurred expense of $0.3 million, $0.2 million, $0.8 million and $0.6 million respectively, for these services. Some of the Company’s expenses are periodically paid by Two River. The Company reimburses Two River for these expenses and no interest is charged on the outstanding balance. These reimbursable expenses totaled $15.0 thousand and $7.4 thousand for three months ended September 30, 2020 and 2019, respectively, and $24.1 thousand and $44.9 thousand for the nine months ended September 30, 2020 and 2019, respectively. As of September 30, 2020 and December 31, 2019, the Company had payables to Two River of $92.5 thousand and $75.0 thousand, respectively. All balances owed as December 31, 2019 were paid in full during the first quarter of 2020. In 2019, the Company entered into a consulting agreement with Bellco Capital, LLC (Bellco) to provide various executive services to the Company. Arie Belldegrun, M.D., FACS, the Chairman of the Board of Directors, is the Chairman of Bellco. Rebecka Belldegrun, M.D., a director of the Company, is the President and Chief Executive Officer of Bellco. During the three months ended September 30, 2020 and 2019 and the nine months ended September 30, 2020 and 2019, the Company incurred expense of $6.3 thousand, $6.3 thousand, $18.9 thousand and $18.9 thousand, respectively, for these services.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2020
Subsequent Events [Abstract]
Subsequent Events	SUBSEQUENT EVENTS In preparing the interim financial statements as of September 30, 2020 for the three and nine months periods then ended, the Company evaluated subsequent events for recognition and measurement purposes. The Company concluded that no events or transactions have occurred that require disclosure in the accompanying financial statements, other than the following. a. Authorization of Preferred Stock On October 14, 2020, the Company amended and restated its certificate of incorporation. Among other things, the amended and restated certification of incorporation authorizes 10,000,000 shares of undesignated preferred stock with a par value of $0.001. b. Increase in Authorization of Common Stock On October 2, 2020, the Company amended its certificate of incorporation to, among other things, increase the number of authorized shares of common stock from 40,000,000 to 200,000,000. c. Amendments to Convertible Preferred Stock On October 1, 2020, the required holders of the Company’s Convertible Preferred Stockholders authorized the automatic conversion of all shares of the Convertible Preferred Stock in the IPO. d. Forward Stock Split On October 2, 2020, the Company effected a 1.055-for-1 forward stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing ratios for each series of the Company’s Preferred Stock. In addition, the number of shares subject to each stock option outstanding was proportionally increased by the forward stock split ratio, and the exercise price per share was proportionally decreased by the forward stock split ratio. Accordingly, all share and per share amounts for all periods presented in the accompanying financial statements and notes thereto have been adjusted retrospectively, where applicable, to reflect this forward stock split and adjustment of the preferred stock conversion ratios. e. 2020 Equity Incentive Plan In October 2020, the Company’s board of directors and stockholders approved the Company’s 2020 Equity Incentive Plan (the 2020 Plan), which became effective on October 8, 2020. The 2020 Plan provides for the issuance of up to 11,938,152 of share-based awards. f. Employee Stock Purchase Plan In October 2020, the Company’s board of directors and stockholders approved the Company’s 2020 Employee Stock Purchase Plan (the ESPP). The ESPP became effective October 8, 2020. The ESPP provides for the issuance of up to 688,000 of share-based awards. g. Conversion of convertible notes In October 2020, upon the closing Company’s IPO, the 2020 Notes automatically converted into 9,610,713 shares of common stock (See Note 15). Upon the closing of the IPO, the Gilead Note was also converted into 188,567 shares of common stock as the result of Gilead’s election (See Note 13). The Company will record a loss of $14.1 million and a gain of $1.9 million in the fourth quarter of fiscal 2020 upon the settlement of the 2020 Notes and Gilead Note, respectively. h. Lease termination In November 2020, the Company entered into a lease termination agreement for its lab space in Cambridge, Massachusetts. The lease will terminate as of January 31, 2021 rather than continue through May 2021. The cost for early termination is a payment of $0.1 million.

Significant Accounting Polici_2

Significant Accounting Policies, Estimates and Judgments (Policies)	9 Months Ended
	Sep. 30, 2020
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 pursuant to Form 10-Q and Article 10 of Regulation S-X of the Securities and Exchange Commission (SEC). Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. Unaudited Interim Financial Information The accompanying balance sheet as of September 30, 2020, the statements of operations and comprehensive loss for the three and nine months ended September 30, 2020 and 2019, the statements of convertible preferred stock and stockholders’ deficit as of September 30, 2020 and 2019, the statements of cash flows for the nine months ended September 30, 2020 and 2019, and the financial data and other financial data and other information disclosed to in the notes to the condensed financial statements are unaudited. The unaudited interim financial statements have been prepared on a basis consistent with the Company’s audited annual financial statements and, in the opinion of management, reflect all adjustments, consisting solely of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of September 30, 2020 and the results of its operations the three and nine months ended September 30, 2020 and 2019. The results for the three and nine months ended September 30, 2020 are not necessarily indicative of results to be expected for the year ending December 31, 2020, any other interim periods, or any future year or period. These financial statements should be read in conjunction with the Company's audited financial statements included in the prospectus dated October 8, 2020 that forms a part of the Company's Registration Statements on Form S-1 (File No. 333-248925), as filed with the SEC pursuant to Rule 424 promulgated under the Securities Act of 1933, as amended, on October 9, 2020.
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, the disclosure of assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods.
Research and Development Expenses	Research and Development Expenses Research and development (R&D) expenses consist primarily of personnel costs, including salaries, benefits and stock-based compensation, discovery research performed by contract research organizations (CROs), materials and supplies, licenses and fees and overhead allocations consisting of various support and facility-related costs. The Company expenses R&D costs as the services are performed or the goods are received. Research and development expenses for the three and nine months ended September 30, 2020 included $6.6 million related to acquired intangible assets as research and development costs pursuant to the Asset Purchase Agreement with Gilead (See Note 13) as, at the time of acquisition of the asset, the technology was under development; was not approved by the U.S. Food and Drug Administration or other regulatory agencies for marketing; had not reached technical feasibility; or otherwise had no foreseeable alternative future use. There were no acquired intangible assets expensed for the three and nine months ended September 30, 2019. CRO costs are a significant component of R&D expenses. The Company monitors levels of performance under each significant contract through communications with its CROs. The Company accrues costs for discovery research performed by CROs over the service periods specified in the contracts and adjusts our estimates, if required, based upon its ongoing review of the level of effort and costs actually incurred by the CROs. All of the Company’s material CRO contracts are terminable by the Company upon written notice and it is generally only liable for actual services completed by the CRO and certain non-cancellable expenses incurred at any point of termination.
Cash and Cash Equivalents and Restricted Cash	Cash and Cash Equivalents The Company considers all highly liquid investments with an original maturity of three months or less from the date of purchase to be cash equivalents. Restricted Cash
Concentration of Credit Risk and Other Risks and Uncertainties	Concentration of Credit Risk and Other Risks and Uncertainties Financial instruments, which potentially subject the Company to significant concentrations of credit risk, consist primarily of cash and cash equivalents and investments. The primary objectives for the Company’s investment portfolio are the preservation of capital and the maintenance of liquidity. The Company does not enter into any investment transaction for trading or speculative purposes. The Company’s investment policy limits investments to certain types of instruments such as certificates of deposit, money market instruments, obligations issued by the U.S. government and U.S. government agencies as well as corporate debt securities, and places restrictions on maturities and concentration by type and issuer. The Company maintains cash balances in excess of amounts insured by the FDIC and concentrated within a limited number of financial institutions. The accounts are monitored by management and management believes that the financial institutions are financially sound, and, accordingly, minimal credit risk exists with respect to these financial institutions. As of September 30, 2020 and December 31, 2019, the Company has not experienced any credit losses in such accounts or investments. The Company is subject to a number of risks common for biopharmaceutical companies, including, but not limited to, dependency on the clinical and commercial success of its product candidates, ability to obtain regulatory approval of its product candidates, the need for substantial additional financing to achieve its goals, uncertainty of broad adoption of its approved products, if any, by physicians and patients, significant competition, and untested manufacturing capabilities.
Investments	Investments Investments are available-for-sale and are carried at estimated fair value. The Company’s valuations of available-for-sale securities are generally derived from independent pricing services based upon quoted prices in active markets for similar securities, with prices adjusted for yield and number of days to maturity, or based on industry models using data inputs, such as interest rates and prices that can be directly observed or corroborated in active markets. Management determines the appropriate classification of its investments in debt securities at the time of purchase. Investments with original maturities beyond three months at the date of purchase and which mature at, or less than 12 months from, the balance sheet date are classified as short-term investments. Unrealized gains and losses are excluded from earnings and are reported as components of comprehensive loss. The Company periodically evaluates whether declines in fair values of its available-for-sale 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 available-for-sale security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not that it will be required to sell any available-for-sale securities before recovery of its amortized cost basis. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on available-for-sale securities are included in interest income (expense), net. The cost of investments sold is based on the specific-identification method. Interest income on investments is included in interest income (expense), net on the Company’s statements of operations and comprehensive loss.
Fair Value Measurement	Fair Value Measurement Assets and liabilities recorded at fair value on a recurring basis in the balance sheets are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements 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. 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. The fair value estimates are made at a discrete point in time based on relevant market information and information about the financial instruments. Fair value estimates are based on judgments regarding future expected loss experience, current economic conditions, risk characteristics of various financial instruments, and other factors. These estimates are subjective in nature and involve uncertainties and matters of significant judgment and, therefore, cannot be determined with precision. Changes in assumptions could significantly affect the estimates. As of September 30, 2020, the Company recorded both financial assets and financial liabilities requiring fair value measurement. The financial assets include cash equivalents and investments. The financial liabilities consist of the convertible promissory note (Gilead Note) from the Gilead Asset Purchase Agreement (See Note 13) and the convertible promissory notes (2020 Notes) sold in August 2020 (See Note 15). As of December 31, 2019, the Company recorded financial assets, including cash equivalents, and investments, requiring fair value measurement. There were no financial liabilities as of December 31, 2019. Fair Value Option Under U.S. GAAP, the Company has the irrevocable option to report most financial assets and financial liabilities at fair value on an instrument by instrument basis, with changes in fair value reported in the statements of operations. The option was elected for the treatment of the Company’s convertible promissory note from the Gilead Asset Purchase Agreement entered into in July 2020 (See Note 13) and the convertible promissory notes (2020 Notes) sold in August 2020 (See Note 15) in order to more accurately reflect the economic value of the notes.
Property and Equipment, Net	Property and Equipment, Net Property and equipment is stated at cost less accumulated depreciation and amortization. Depreciation and amortization are recognized using the straight-line method over the estimated useful lives of the respective assets . Leasehold improvements are amortized over the lesser of their useful lives or the remaining life of the lease. When assets are retired or otherwise disposed of, the cost and related accumulated depreciation and amortization are removed from the balance sheet and the resulting gain or loss is reflected in operations in the period realized . Repairs and maintenance costs are expensed as incurred. Estimated useful lives in years are generally as follows: Description Estimated Useful Life Lab equipment 3 to 7 years Leasehold improvements Shorter of useful life or lease term
Deferred Offering Costs	Deferred Offering CostsOffering costs, including legal, accounting, and filing fees related to the IPO, were deferred and were offset against the offering proceeds upon the completion of the IPO.
Leases	Leases The Company determines if an arrangement is a lease at inception. Operating leases are included in operating lease right-of-use (ROU) assets, current portion of operating lease liabilities, and noncurrent operating lease liabilities on the Company’s balance sheet. Finance leases are included in property and equipment, current portion of other liabilities, and other noncurrent liabilities on the balance sheet.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets, including property and equipment, are reviewed for impairment whenever facts or circumstances either internally or externally may suggest that the carrying value of an asset or asset group may not be recoverable. Should there be an indication of impairment, the Company tests for recoverability by comparing the estimated undiscounted future cash flows expected to result from the use of the asset or asset group and its eventual disposition to the carrying amount of the asset or asset group. Any excess of the carrying value of the asset or asset group over its estimated fair value is recognized as an impairment loss.
Stock-Based Compensation	Stock-Based Compensation The Company measures stock-based awards granted to employees and nonemployees based on the fair value on the date of the grant and recognizes stock-based compensation expense of those awards over the requisite service period, which is generally the vesting period of the respective award. The Company calculates the fair value measurement of stock options using the Black-Scholes option-pricing model.
Income Taxes	Income Taxes The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the Company’s financial statements or tax returns. Under this method, deferred tax assets and liabilities are determined based on differences between the financial statement carrying amounts and the tax bases of the assets and liabilities using the enacted tax rates in effect in the years in which the differences are expected to reverse. A valuation allowance against deferred tax assets is recorded if, based on the weight of the available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. The Company accounts for uncertainty in income taxes recognized in the financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the three and nine months ended September 30, 2020, other comprehensive loss consisted of unrealized gains and losses from available-for-sale securities. There was no difference between net loss and comprehensive loss for the three and nine months ended September 30, 2019.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is computed using the weighted-average number of shares of common stock outstanding during the period. Diluted net loss per share is computed using the sum of the weighted-average number of shares of common stock outstanding during the period and the effect of dilutive securities. In periods in which the Company reports a net loss, diluted net loss per share is the same as basic net loss per share, since dilutive shares of common stock are not assumed to have been issued if their effect on net loss per share is anti-dilutive.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In August 2018, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2018-13, Fair Value Measurement (Topic 820) Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement (ASU 2018-13). The Company adopted ASU No. 2018-13 on January 1, 2019. This standard modifies certain disclosure requirements on fair value measurements. The adoption of this standard did not have a material impact on the Company’s financial statements. In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows, Restricted Cash (ASU 2016-18). This ASU requires changes in restricted cash during the period to be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. If cash, cash equivalents and restricted cash are presented in more than one line item on the balance sheet, the new guidance requires a reconciliation of the total in the statement of cash flows to the related captions in the balance sheet. The Company elected to adopt ASU 2016-18 on January 1, 2019 and has reflected the adoption in its financial statements. A reconciliation of the cash, cash equivalents, and restricted cash reported within the balance sheet that sum to the total of the same amounts shown in the statement of cash flows is as follows: September 30, 2020 2019 (in thousands) Cash and cash equivalents $ 156,196 $ 102,738 Restricted cash 2,026 — Total cash, cash equivalents and restricted cash as shown on the statements of cash flows $ 158,222 $ 102,738 In addition, the Company adopted ASU No 2016-15, Statement of Cash Flow (ASU 2016-15) in 2019. The guidance reduces diversity in how certain cash receipts and cash payments are presented and classified in the statements of cash flows. The adoption of ASU 2016-15 did not have a material impact on the Company’s condensed financial statements. In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses: Measurement of Credit Losses on Financial Instruments and also issued subsequent amendments to the initial guidance: ASU 2018-19, ASU 2019-04, ASU 2019-05, and ASU 2019-11. The standard requires measurement and recognition of expected credit losses for financial assets by requiring an allowance to be recorded as an offset to the amortized cost of such assets. For available-for-sale debt securities, expected credit losses should be estimated when the fair value of the debt securities is below their associated amortized costs. The standard will become effective for the Company in the first quarter of 2020, with early adoption permitted beginning the first quarter of 2019. The modified retrospective approach should be applied upon adoption of this new guidance. The Company’s financial instruments that are in the scope of ASU 2016-13 include available-for-sale debt securities. The Company adopted this standard on January 1, 2020 and this amendment did not have a material impact on its financial statements. In December 2019, the FASB issued ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes (ASU 2019-12), which simplifies the accounting for income taxes, eliminates certain exceptions within ASC 740, Income Taxes

Significant Accounting Polici_3

Significant Accounting Policies, Estimates and Judgments (Tables)	9 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Property, Plant and Equipment	Estimated useful lives in years are generally as follows: Description Estimated Useful Life Lab equipment 3 to 7 years Leasehold improvements Shorter of useful life or lease term Property and equipment, net consisted of the following as of September 30, 2020 and December 31, 2019: September 30, 2020 December 31, 2019 (in thousands) Property and equipment: Lab equipment $ 5,256 $ 3,978 Finance lease on R&D equipment 139 139 Construction in progress 5,891 50 Total property and equipment 11,286 4,167 Less: Accumulated depreciation and amortization (985) (446) Total property and equipment, net $ 10,301 $ 3,721
Schedule of Cash, Cash Equivalents, and Restricted Cash Equivalents	A reconciliation of the cash, cash equivalents, and restricted cash reported within the balance sheet that sum to the total of the same amounts shown in the statement of cash flows is as follows: September 30, 2020 2019 (in thousands) Cash and cash equivalents $ 156,196 $ 102,738 Restricted cash 2,026 — Total cash, cash equivalents and restricted cash as shown on the statements of cash flows $ 158,222 $ 102,738

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2020
Fair Value Disclosures [Abstract]
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis	Financial assets and liabilities subject to fair value measurements on a recurring basis and the level of inputs used in such measurements by major security type as of September 30, 2020 and December 31, 2019 were as follows: September 30, 2020 Level 1 Level 2 Level 3 Fair Value (in thousands) Financial Assets: Money market funds $ 84,635 $ — $ — $ 84,635 Certificates of deposit 1,234 — — 1,234 Corporate bonds — 18,021 — 18,021 U.S. treasury securities 40,926 — — 40,926 Total financial assets $ 126,795 $ 18,021 $ — $ 144,816 Financial Liabilities: 2020 Notes $ — $ — $ 168,506 $ 168,506 Gilead Note — — 5,488 5,488 Total financial liabilities $ — $ — $ 173,994 $ 173,994 December 31, 2019 Level 1 Level 2 Level 3 Fair Value (in thousands) Financial Assets: Money market funds $ 112 $ — $ — $ 112 Certificates of deposit 1,715 — — 1,715 Commercial paper — 4,489 — 4,489 Corporate bonds — 26,432 — 26,432 U.S. agency securities — 1,499 — 1,499 U.S. treasury securities 36,880 — — 36,880 Total financial assets $ 38,707 $ 32,420 $ — $ 71,127 The carrying amounts of accounts payable and accrued expenses approximate their fair values due to their short-term maturities. 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. Both observable and unobservable inputs were used to determine the fair value of positions that the Company has classified within the Level 3 category as of September 30, 2020. Recorded losses associated within the Level 3 category include changes to fair value and are recorded within change in fair value of convertible notes in the statement of operations. Refer to Note 13 and Note 15 for the fair value determination of the Level 3 financial liabilities. The Company did not have any financial assets or liabilities as of December 31, 2019 that required Level 3 inputs. There were no transfers of assets between the fair value measurement levels during any of the periods presented in the accompanying financial statements.

Investments (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2020
Investments, Debt and Equity Securities [Abstract]
Investments Classified by Contractual Maturity Date	The fair value and amortized cost of available-for-sale securities by major security type as of September 30, 2020 and December 31, 2019 were as follows: September 30, 2020 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market funds $ 84,635 $ — $ — $ 84,635 Certificates of deposit 1,225 9 — 1,234 Corporate bonds 18,011 18 (8) 18,021 U.S. treasury securities 40,898 30 (2) 40,926 Total cash equivalents and investments $ 144,769 $ 57 $ (10) $ 144,816 December 31, 2019 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market funds $ 112 $ — $ — $ 112 Certificates of deposit 1,715 — — 1,715 Commercial paper 4,490 — (1) 4,489 Corporate bonds 26,444 1 (13) 26,432 U.S. agency securities 1,500 — (1) 1,499 U.S. treasury securities 36,884 1 (5) 36,880 Total cash equivalents and investments $ 71,145 $ 2 $ (20) $ 71,127 These available-for-sale securities were classified on the Company’s balance sheets as of September 30, 2020 and December 31, 2019 as: Fair Value September 30, 2020 December 31, 2019 (in thousands) Cash equivalents $ 84,973 $ 6,751 Short-term investments 55,069 59,614 Long-term investments 4,774 4,762 Total cash equivalents and investments $ 144,816 $ 71,127 The fair values of available-for-sale securities by contractual maturity as of September 30, 2020 and December 31, 2019 were as follows: September 30, 2020 December 31, 2019 (in thousands) Due in 1 year or less $ 55,407 $ 66,253 Due in 1 to 2 years 4,774 4,762 Instruments not due at a single maturity date 84,635 112 Total cash equivalents and investments $ 144,816 $ 71,127

Prepaid Expenses And Other Cu_2

Prepaid Expenses And Other Current Assets (Tables)	9 Months Ended
	Sep. 30, 2020
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Schedule of Prepaid Expenses and Other Assets	Prepaid expenses and other current assets consisted of the following as of September 30, 2020 and December 31, 2019: September 30, 2020 December 31, 2019 (in thousands) Accrued interest on short-term available-for-sale securities $ 217 $ 198 Prepaid equipment service contracts 432 191 Prepaid external research and development 410 113 Prepaid software 254 180 Prepaid insurance 36 23 Prepaid rent 3 196 Other prepaid expenses 276 218 Total prepaid expenses and other current assets $ 1,628 $ 1,119

Property, Plant, and Equipment

Property, Plant, and Equipment (Tables)	9 Months Ended
	Sep. 30, 2020
Property, Plant and Equipment [Abstract]
Property and Equipment [Table Text Block]	Estimated useful lives in years are generally as follows: Description Estimated Useful Life Lab equipment 3 to 7 years Leasehold improvements Shorter of useful life or lease term Property and equipment, net consisted of the following as of September 30, 2020 and December 31, 2019: September 30, 2020 December 31, 2019 (in thousands) Property and equipment: Lab equipment $ 5,256 $ 3,978 Finance lease on R&D equipment 139 139 Construction in progress 5,891 50 Total property and equipment 11,286 4,167 Less: Accumulated depreciation and amortization (985) (446) Total property and equipment, net $ 10,301 $ 3,721

Accrued Expenses And Current _2

Accrued Expenses And Current Portion Of Other Liabilities (Tables)	9 Months Ended
	Sep. 30, 2020
Payables and Accruals [Abstract]
Schedule of Accrued Liabilities	Accrued expenses consisted of the following as of September 30, 2020 and December 31, 2019: September 30, 2020 December 31, 2019 (in thousands) Accrued compensation $ 1,107 $ 528 Accrued franchise tax 38 43 External research and development 469 241 Accrued offering costs 83 — Other accrued expenses 83 6 Total accrued expenses $ 1,780 $ 818
Other Current Liabilities	Current portion of other liabilities consist of the following as of September 30, 2020 and December 31, 2019: September 30, 2020 December 31, 2019 (in thousands) Current portion of finance lease liability $ 14 $ 32 Current portion of unvested early exercised share liability 2,123 56 Total current portion of other current liabilities $ 2,137 $ 88

Preferred Stock (Tables)

Preferred Stock (Tables)	9 Months Ended
	Sep. 30, 2020
Temporary Equity Disclosure [Abstract]
Preferred Stock	The Preferred Stock (as defined below) consisted of the following as of September 30, 2020 and December 31, 2019: September 30, 2020 Preferred Stock Authorized Preferred Carrying Liquidation Common (in thousands, except share amounts) Series Seed Preferred Stock 7,806,977 7,806,977 $ 17,985 $ 18,016 8,236,347 Series A Preferred Stock 13,700,000 13,697,916 104,922 105,000 14,451,278 Total 21,506,977 21,504,893 $ 122,907 $ 123,016 22,687,625 December 31, 2019 Preferred Stock Authorized Preferred Carrying Liquidation Common (in thousands, except share amounts) Series Seed Preferred Stock 7,806,977 7,806,977 $ 17,985 $ 18,016 8,236,347 Series A Preferred Stock 13,700,000 13,697,916 104,922 105,000 14,451,278 Total 21,506,977 21,504,893 $ 122,907 $ 123,016 22,687,625

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2020
Share-based Payment Arrangement [Abstract]
Schedule of Option-pricing Model Assumptions	The Black-Scholes option-pricing model assumptions that the Company used to determine the grant-date fair value of stock options for the three and nine months ended September 30, 2020 and 2019 were as follows, presented on a weighted-average basis: Three months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 Fair value of common stock per share $ 6.79 $ 2.53 $ 4.70 $ 1.35 Expected term (in years) 5.99 6.03 5.99 6.06 Expected volatility 81.72% 66.88% 76.93% 76.97% Risk-free interest rate 0.36% 1.67% 0.73% 2.15% Expected dividend —% —% —% —%
Stock Option Activity	Stock option activity under the 2017 Plan as of September 30, 2020 is summarized as follows: Number of Options Weighted-Average Exercise Price Weighted-Average Remaining Contractual Term Aggregate Intrinsic Value (in years) (in thousands) Balance at December 31, 2019 1,726,233 $ 1.19 9.05 $ 27,287 Granted 4,015,631 4.70 Forfeited (117,921) 0.76 Exercised (406,057) 1.01 Balance at September 30, 2020 5,217,886 $ 3.91 9.34 $ 68,291 Exercisable at September 30, 2020 69,910 $ 1.33 8.43 $ 1,096 Unvested and expected to vest at September 30, 2020 5,147,976 $ 3.95 9.36 $ 67,195
Stock-based Compensation, Classified in Statements of Operations	Stock-based compensation expense was classified in the Company’s statements of operations and comprehensive loss for the three and nine months ended September 30, 2020 and 2019 as follows: Three months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 (in thousands) Research and development expenses $ 246 $ 9 $ 541 $ 20 General and administrative expenses 1,007 14 1,193 40 Total stock-based compensation expense $ 1,253 $ 23 $ 1,734 $ 60

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2020
Earnings Per Share [Abstract]
Schedule of Earnings Per Share, Basic and Diluted	The following table summarizes the computation of basic and diluted net loss per share of the Company for the three and nine months ended September 30, 2020 and 2019: Three months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 (in thousands, except share and per share amounts) Net loss $ (39,713) $ (4,013) $ (55,287) $ (10,652) Weighted-average common stock outstanding, basic and diluted 6,127,146 5,409,433 5,886,191 5,196,681 Net loss per share, basic and diluted $ (6.48) $ (0.74) $ (9.39) $ (2.05)
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share	The Company excluded the following potential shares of common stock, presented based on amounts outstanding at each stated period end, from the computation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect: September 30, 2020 September 30, 2019 Convertible preferred stock 22,687,625 22,687,625 2020 Notes 10,741,406 — Gilead Note 210,752 — Stock options to purchase common stock 3,359,622 389,086 Early exercised stock options subject to future vesting 1,858,264 854,123 Restricted stock award subject to future vesting 271,683 242,123 Total 39,129,352 24,172,957

Commitment and Contingencies (T

Commitment and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2020
Commitments and Contingencies Disclosure [Abstract]
Key Valuation Assumptions	Key valuation assumptions are as follows: As of September 30, 2020 Fundraise scenario IPO cash payout scenario IPO conversion scenario Liquidation cash payout scenario Liquidation conversion scenario Remaining term (years) 1.78 0.08 0.08 1.79 1.79 Discount rate 24.2 % 24.2 % 24.2 % 24.2 % 24.2 % Weighting 1.5 % 95.0 % — % 2.0 % 1.5 % The Company determined the fair value of the Gilead Note at inception by determining the present value of the Gilead Note if they were to settle either upon closing of an IPO, a deemed liquidation, or a qualified or non-qualified equity fundraise. Key valuation assumptions are as follows: At inception Fundraise scenario IPO cash payout scenario IPO conversion scenario Liquidation cash payout scenario Liquidation conversion scenario Remaining term (years) 2.00 0.30 0.30 2.00 2.00 Discount rate 24.2 % 24.2 % 24.2 % 24.2 % 24.2 % Weighting 18.6 % 12.9 % — % 50.0 % 18.6 % As of September 30, 2020 IPO Scenario Series B Scenario Financing Scenario Remaining term (years) 0.08 1.39 1.39 Discount rate 24.2 % 24.2 % 24.2 % Weighting 95.0 % 2.5 % 2.5 %

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2020
Leases [Abstract]
Assets and Liabilities, Finance Leases, Lessee	The following table summarizes the presentation of the Company’s finance lease in its balance sheets as of September 30, 2020 and December 31, 2019: Balance Sheet Caption September 30, 2020 December 31, 2019 (in thousands) Assets: Property and equipment, net $ 88 $ 103 Liabilities: Current portion of other liabilities 14 32 Other noncurrent liabilities — 5 Total finance lease liabilities $ 14 $ 37
Assets And Liabilities, Operating Leases, Lessee	The following table summarizes the presentation of the Company’s operating leases in its balance sheets as of September 30, 2020 and December 31, 2019: Balance Sheet Caption September 30, 2020 December 31, 2019 (in thousands) Assets: Operating lease assets $ 29,355 $ 473 Liabilities: Current portion of operating lease liabilities $ 1,300 $ 285 Noncurrent operating lease liabilities 29,484 211 Total operating lease liabilities $ 30,784 $ 496
Summary of Lease Costs	The following table summarizes the effect of finance lease costs in the Company’s statements of operations and comprehensive loss for the three and nine months ended September 30, 2020 and 2019: Statement of Operations and Comprehensive Loss Caption Three months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 (in thousands) Research and development $ 5 $ 5 $ 15 $ 15 Interest income (expense), net — 1 1 3 Total finance lease cost $ 5 $ 6 $ 16 $ 18 The following table summarizes the effect of operating lease costs in the Company’s statements of operations and comprehensive loss for the three and nine months ended September 30, 2020 and 2019 : Statement of Operations and Comprehensive Loss Caption Three months ended September 30, Nine months ended September 30, 2020 2019 2020 2019 (in thousands) Research and development $ 1,397 $ — $ 2,843 $ — General and administrative 388 78 890 233 Total operating lease cost $ 1,785 $ 78 $ 3,733 $ 233
Schedule of Maturities on Lease Liabilities	The undiscounted future non-cancellable lease payments under the Company's operating leases as of September 30, 2020 for the next five years and thereafter is expected to be as follows: Period Ending December 31, Amount (in thousands) Remaining three months of 2020 $ 1,329 2021 5,770 2022 4,789 2023 4,933 2024 and thereafter 35,915 Total undiscounted lease payments 52,736 Less: Present value adjustment (18,238) Less: Tenant improvement allowance (3,714) Present value of operating lease liabilities $ 30,784

2020 Notes (Tables)

2020 Notes (Tables)	9 Months Ended
	Sep. 30, 2020
Debt Disclosure [Abstract]
Key Valuation Assumptions	Key valuation assumptions are as follows: As of September 30, 2020 Fundraise scenario IPO cash payout scenario IPO conversion scenario Liquidation cash payout scenario Liquidation conversion scenario Remaining term (years) 1.78 0.08 0.08 1.79 1.79 Discount rate 24.2 % 24.2 % 24.2 % 24.2 % 24.2 % Weighting 1.5 % 95.0 % — % 2.0 % 1.5 % The Company determined the fair value of the Gilead Note at inception by determining the present value of the Gilead Note if they were to settle either upon closing of an IPO, a deemed liquidation, or a qualified or non-qualified equity fundraise. Key valuation assumptions are as follows: At inception Fundraise scenario IPO cash payout scenario IPO conversion scenario Liquidation cash payout scenario Liquidation conversion scenario Remaining term (years) 2.00 0.30 0.30 2.00 2.00 Discount rate 24.2 % 24.2 % 24.2 % 24.2 % 24.2 % Weighting 18.6 % 12.9 % — % 50.0 % 18.6 % As of September 30, 2020 IPO Scenario Series B Scenario Financing Scenario Remaining term (years) 0.08 1.39 1.39 Discount rate 24.2 % 24.2 % 24.2 % Weighting 95.0 % 2.5 % 2.5 %

Organization, Consolidation and

Organization, Consolidation and Presentation of Financial Statements (Details) $ / shares in Units, $ in Thousands	Oct. 02, 2020	Oct. 31, 2020USD ($)$ / sharesshares	Sep. 30, 2020USD ($)shares	Jun. 30, 2020USD ($)shares	Mar. 31, 2020USD ($)shares	Sep. 30, 2019USD ($)shares	Jun. 30, 2019USD ($)shares	Mar. 31, 2019USD ($)shares	Sep. 30, 2020USD ($)segmentshares	Sep. 30, 2019USD ($)shares	Sep. 30, 2020USD ($)shares	Dec. 31, 2019USD ($)shares	Dec. 31, 2018shares
Subsidiary, Sale of Stock [Line Items]
Number of operating segments | segment									1
Convertible preferred stock, shares outstanding (in shares)			21,504,893	21,504,893	21,504,893	21,504,893	14,949,465	7,806,977	21,504,893	21,504,893	21,504,893	21,504,893	7,806,977
Deferred offering costs | $			$ 2,900						$ 2,900		$ 2,900
Cash, cash equivalents, and short-term investments | $			144,816						144,816		144,816	$ 71,127
Net loss | $			$ 39,713	$ 8,579	$ 6,995	$ 4,013	$ 3,509	$ 3,130	$ 55,287	$ 10,652	$ 78,700
Common Stock | Subsequent Event
Subsidiary, Sale of Stock [Line Items]
Shares issued upon conversion of convertible preferred stock (in shares)		22,687,625
Forward stock split ratio	1.055
IPO | Subsequent Event
Subsidiary, Sale of Stock [Line Items]
Shares issued in private placement (in shares)		15,131,579
Shares issued in private placement, price per share (in dollars per share) | $ / shares		$ 19
Net proceeds from stock offering | $		$ 263,900
Payments of underwriting discounts and commissions | $		(20,100)
Payments of offering costs | $		$ 3,500
Conversion of stock, shares converted (in shares)		21,504,893
Convertible preferred stock, shares outstanding (in shares)		0
IPO | Common Stock | Subsequent Event
Subsidiary, Sale of Stock [Line Items]
Shares issued upon conversion of convertible preferred stock (in shares)		22,687,625
IPO | Common Stock | Subsequent Event | 2020 Convertible Notes
Subsidiary, Sale of Stock [Line Items]
Shares issued upon conversion of convertible preferred stock (in shares)		9,610,713
IPO | Common Stock | Subsequent Event | 2020 Gilead Notes
Subsidiary, Sale of Stock [Line Items]
Shares issued upon conversion of convertible preferred stock (in shares)		188,567
Over-Allotment Option | Subsequent Event
Subsidiary, Sale of Stock [Line Items]
Shares issued in private placement (in shares)		1,973,684

Significant Accounting Polici_4

Significant Accounting Policies, Estimates and Judgments - R&D Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Schedule Of Asset Acquisition [Line Items]
Research and development	$ 16,755	$ 3,526	$ 30,125	$ 8,698
Gilead Asset Purchase
Schedule Of Asset Acquisition [Line Items]
Research and development	$ 6,600		$ 6,600

Significant Accounting Polici_5

Significant Accounting Policies, Estimates and Judgments - Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019		Sep. 30, 2019
Accounting Policies [Abstract]
Restricted cash	$ 2,026	$ 0	[1]	$ 0
[1]	The balance sheet as of December 31, 2019 is derived from the audited financial statements as of that date.

Significant Accounting Polici_6

Significant Accounting Policies, Estimates and Judgments - Property, Plant and Equipment (Details) - Lab equipment	9 Months Ended
	Sep. 30, 2020
Minimum
Property, Plant and Equipment [Line Items]
Useful life	3 years
Maximum
Property, Plant and Equipment [Line Items]
Useful life	7 years

Significant Accounting Polici_7

Significant Accounting Policies, Estimates and Judgments - Schedule of Cash, Cash Equivalents, and Restricted Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019		Sep. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Cash and cash equivalents	$ 156,196	$ 32,570	[1]	$ 102,738
Restricted cash	2,026	0	[1]	0
Total cash, cash equivalents and restricted cash as shown on the statements of cash flows	$ 158,222	$ 32,570		$ 102,738	$ 10,226
[1]	The balance sheet as of December 31, 2019 is derived from the audited financial statements as of that date.

Fair Value Measurements (Detail

Fair Value Measurements (Details) - Fair Value, Recurring - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets	$ 144,816	$ 71,127
Total financial liabilities	173,994
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	84,635	112
Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,234	1,715
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		4,489
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	18,021	26,432
U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		1,499
U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	40,926	36,880
2020 Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
2020 Notes	168,506
Gilead Note
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gilead Note	5,488
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets	126,795	38,707
Total financial liabilities	0
Level 1 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	84,635	112
Level 1 | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,234	1,715
Level 1 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0
Level 1 | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Level 1 | U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0
Level 1 | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	40,926	36,880
Level 1 | 2020 Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
2020 Notes	0
Level 1 | Gilead Note
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gilead Note	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets	18,021	32,420
Total financial liabilities	0
Level 2 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0	0
Level 2 | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Level 2 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		4,489
Level 2 | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	18,021	26,432
Level 2 | U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		1,499
Level 2 | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Level 2 | 2020 Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
2020 Notes	0
Level 2 | Gilead Note
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gilead Note	0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets	0	0
Total financial liabilities	173,994
Level 3 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0	0
Level 3 | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Level 3 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0
Level 3 | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Level 3 | U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0
Level 3 | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	$ 0
Level 3 | 2020 Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
2020 Notes	168,506
Level 3 | Gilead Note
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gilead Note	$ 5,488

Investments - Fair Value and Am

Investments - Fair Value and Amortized Cost (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Money market funds, amortized cost	$ 84,973	$ 6,751
Total cash equivalents and investments, amortized cost	144,769	71,145
Total cash equivalents and investments, unrealized gains	57	2
Total cash equivalents and investments, unrealized losses	(10)	(20)
Total cash equivalents and investments, fair value	144,816	71,127
Money market funds
Schedule of Held-to-maturity Securities [Line Items]
Money market funds, amortized cost	84,635	112
Certificates of deposit
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,225	1,715
Unrealized Gains	9	0
Unrealized Losses	0	0
Fair Value	1,234	1,715
Commercial paper
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		4,490
Unrealized Gains		0
Unrealized Losses		(1)
Fair Value		4,489
Corporate bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	18,011	26,444
Unrealized Gains	18	1
Unrealized Losses	(8)	(13)
Fair Value	18,021	26,432
U.S. agency securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		1,500
Unrealized Gains		0
Unrealized Losses		(1)
Fair Value		1,499
U.S. treasury securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	40,898	36,884
Unrealized Gains	30	1
Unrealized Losses	(2)	(5)
Fair Value	$ 40,926	$ 36,880

Investments - Balance Sheet (De

Investments - Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Cash equivalents	$ 84,973	$ 6,751
Short-term investments	55,069	59,614	[1]
Long-term investments	4,774	4,762	[1]
Total cash equivalents and investments	$ 144,816	$ 71,127
[1]	The balance sheet as of December 31, 2019 is derived from the audited financial statements as of that date.

Investments - Contractual Matur

Investments - Contractual Maturity (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Due in 1 year or less	$ 55,407	$ 66,253
Due in 1 to 2 years	4,774	4,762
Instruments not due at a single maturity date	84,635	112
Total cash equivalents and investments	$ 144,816	$ 71,127
Available-for-sale, remaining contractual maturity	23 months	18 months

Prepaid Expenses And Other Cu_3

Prepaid Expenses And Other Current Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Accrued interest on short-term available-for-sale securities	$ 217	$ 198
Prepaid equipment service contracts	432	191
Prepaid external research and development	410	113
Prepaid software	254	180
Prepaid insurance	36	23
Prepaid rent	3	196
Other prepaid expenses	276	218
Prepaid expenses and other current assets	$ 1,628	$ 1,119	[1]
[1]	The balance sheet as of December 31, 2019 is derived from the audited financial statements as of that date.

Property, Plant, and Equipmen_2

Property, Plant, and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Finance lease on R&D equipment	$ 139		$ 139		$ 139
Total property and equipment	11,286		11,286		4,167
Less: Accumulated depreciation and amortization	(985)		(985)		(446)
Total property and equipment, net	10,301		10,301		3,721	[1]
Depreciation and amortization	200	$ 100	539	$ 215
Lab equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	5,256		5,256		3,978
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 5,891		$ 5,891		$ 50
[1]	The balance sheet as of December 31, 2019 is derived from the audited financial statements as of that date.

Accrued Expenses And Current _3

Accrued Expenses And Current Portion Of Other Liabilities - Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Accrued compensation	$ 1,107	$ 528
Accrued franchise tax	38	43
External research and development	469	241
Accrued offering costs	83	0
Other accrued expenses	83	6
Total accrued expenses	$ 1,780	$ 818	[1]
[1]	The balance sheet as of December 31, 2019 is derived from the audited financial statements as of that date.

Accrued Expenses And Current _4

Accrued Expenses And Current Portion Of Other Liabilities - Current Other Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Current portion of finance lease liability	$ 14	$ 32
Current portion of unvested early exercised share liability	2,123	56
Total current portion of other current liabilities	$ 2,137	$ 88	[1]
[1]	The balance sheet as of December 31, 2019 is derived from the audited financial statements as of that date.

Preferred Stock - Narrative (De

Preferred Stock - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	Jul. 01, 2019	May 22, 2018	Oct. 31, 2020	Sep. 30, 2020	Sep. 30, 2019
Temporary Equity [Line Items]
Issuance costs				$ 2,248	$ 0
Common Stock | Subsequent Event
Temporary Equity [Line Items]
Shares issued upon conversion of convertible preferred stock (in shares)			22,687,625
Series Seed Preferred Stock | Convertible Notes
Temporary Equity [Line Items]
Shares issued in private placement, price per share (in dollars per share)		$ 2.30769
Series Seed Preferred Stock | Private Placement
Temporary Equity [Line Items]
Shares issued in private placement (in shares)		7,806,977
Gross proceeds received in private placement		$ 18,000
Series A Preferred Stock | Private Placement
Temporary Equity [Line Items]
Shares issued in private placement (in shares)	13,697,916
Gross proceeds received in private placement	$ 105,000
Shares issued in private placement, price per share (in dollars per share)	$ 7.6654
Issuance costs	$ 100

Preferred Stock (Details)

Preferred Stock (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019		Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Temporary Equity [Line Items]
Preferred Stock Authorized	21,506,977			21,506,977
Preferred Stock Issued	21,504,893			21,504,893
Preferred Stock Outstanding	21,504,893	21,504,893	21,504,893	21,504,893		21,504,893	14,949,465	7,806,977	7,806,977
Carrying Value	$ 122,907	$ 122,907	$ 122,907	$ 122,907	[1]	$ 122,907	$ 72,665	$ 17,985	$ 17,985
Liquidation Preference	$ 123,016			$ 123,016
Common Stock Issuable Upon Conversion	22,687,625			22,687,625
Series Seed Preferred Stock
Temporary Equity [Line Items]
Preferred Stock Authorized	7,806,977			7,806,977
Preferred Stock Issued	7,806,977			7,806,977
Preferred Stock Outstanding	7,806,977			7,806,977
Carrying Value	$ 17,985			$ 17,985
Liquidation Preference	$ 18,016			$ 18,016
Common Stock Issuable Upon Conversion	8,236,347			8,236,347
Series A Preferred Stock
Temporary Equity [Line Items]
Preferred Stock Authorized	13,700,000			13,700,000
Preferred Stock Issued	13,697,916			13,697,916
Preferred Stock Outstanding	13,697,916			13,697,916
Carrying Value	$ 104,922			$ 104,922
Liquidation Preference	$ 105,000			$ 105,000
Common Stock Issuable Upon Conversion	14,451,278			14,451,278
[1]	The balance sheet as of December 31, 2019 is derived from the audited financial statements as of that date.

Common Stock (Details)

Common Stock (Details) - $ / shares	Sep. 30, 2020	Dec. 31, 2019	Jul. 01, 2019
Equity [Abstract]
Common stock, shares authorized (in shares)	40,000,000	40,000,000	40,000,000
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001	$ 0.001
Common stock, shares issued (in shares)	6,284,326	5,660,391
Common stock, shares outstanding (in shares)	6,284,326	5,660,391

Stock-Based Compensation - Narr

Stock-Based Compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant-date fair value, options granted (in dollars per share)	$ 4.66	$ 1.54	$ 3.16	$ 0.88
Weighted-average grant-date fair value, options vested (in dollars per share)			$ 0.69	$ 0.09
Stock options exercised, intrinsic value	$ 1,500	$ 200	$ 2,000	$ 400
Stock options, unrecognized cost	$ 14,100		$ 14,100
Early exercised stock options, shares	850,937	60,924	1,551,361	60,924
Current liabilities related to shares that were subject to repurchase	$ 2,123		$ 2,123		$ 56
Current portion of other liabilities
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Current liabilities related to shares that were subject to repurchase	2,100		2,100		100
Other noncurrent liabilities
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-current liabilities related to shares that were subject to repurchase	$ 2,800		$ 2,800		$ 100
2017 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period			10 years
Number of shares that may have been issued (in shares)	6,857,500		6,857,500		3,692,500
Number of shares reserved for future issuance (in shares)	358,596		358,596		1,365,647
2017 Equity Incentive Plan | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			4 years
2017 Equity Incentive Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			10 years
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options recognized, weighted average period			3 years 6 months 10 days

Stock-Based Compensation - Opti

Stock-Based Compensation - Option-pricing Model Assumptions (Details) - Stock options - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of common stock per share (dollars per share)	$ 6.79	$ 2.53	$ 4.70	$ 1.35
Expected term (in years)	5 years 11 months 26 days	6 years 10 days	5 years 11 months 26 days	6 years 21 days
Expected volatility	81.72%	66.88%	76.93%	76.97%
Risk-free interest rate	0.36%	1.67%	0.73%	2.15%
Expected dividend	0.00%	0.00%	0.00%	0.00%

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Stock Option Activity Under the 2017 Plan [Roll Forward]
Balance at December 31, 2019 (in shares)	1,726,233
Granted (in shares)	4,015,631
Forfeited (in shares)	(117,921)
Exercised (in shares)	(406,057)
Balance at September 30, 2020 (in shares)	5,217,886	1,726,233
Exercisable at September 30, 2020 (in shares)	69,910
Unvested and expected to vest at September 30, 2020 (in shares)	5,147,976
Stock Options, Weighted Average Exercise Price [Roll Forward]
Balance at December 31, 2019 (dollars per share)	$ 1.19
Granted (dollars per share)	4.70
Forfeited (dollars per share)	0.76
Exercised (dollars per share)	1.01
Balance at September 30, 2020 (dollars per share)	3.91	$ 1.19
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price	1.33
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Exercise Price	$ 3.95
Stock Option Activity, Additional Disclosures [Abstract]
Balance, Weighted-Average Remaining Contractual Term	9 years 4 months 2 days	9 years 18 days
Exercisable, Weighted-Average Remaining Contractual Term	8 years 5 months 4 days
Unvested and expected to vest, Weighted-Average Remaining Contractual Term	9 years 4 months 9 days
Balance, Aggregate Intrinsic Value	$ 68,291	$ 27,287
Exercisable, Aggregate Intrinsic Value	1,096
Unvested and expected to vest, Aggregate Intrinsic Value	$ 67,195

Stock-Based Compensation - Clas

Stock-Based Compensation - Classified in Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 1,253	$ 23	$ 1,734	$ 60
Research and development expenses
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	246	9	541	20
General and administrative expenses
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 1,007	$ 14	$ 1,193	$ 40

Stock-Based Compensation - Rest

Stock-Based Compensation - Restricted Stock (Details) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		$ 1,253,000	$ 23,000	$ 1,734,000	$ 60,000
Restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock awards outstanding (in shares)		271,683		271,683
Restricted stock | Non-Employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock awards issued (in shares)						538,050
Grant date fair value (in dollars per share)						$ 0.05
Stock-based compensation expense		$ 599,100	$ 1,300	$ 601,700	$ 3,800
Restricted stock | Executive Employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock awards issued (in shares)				274,341
Grant date fair value (in dollars per share)	$ 4.14

Income Taxes (Details)

Income Taxes (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Income tax expense	$ 0	$ 0	$ 0	$ 0

Net Loss Per Share (Details)

Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended		40 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020
Earnings Per Share [Abstract]
Net loss	$ (39,713)	$ (8,579)	$ (6,995)	$ (4,013)	$ (3,509)	$ (3,130)	$ (55,287)	$ (10,652)	$ (78,700)
Weighted-average shares of common stock, basic and diluted (in shares)	6,127,146			5,409,433			5,886,191	5,196,681
Net loss per share, basic and diluted (in dollars per share)	$ (6.48)			$ (0.74)			$ (9.39)	$ (2.05)

Net Loss Per Share - Antidiluti

Net Loss Per Share - Antidilutive Securities Excluded from Computation of Earnings Per Share (Details) - shares	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount (in shares)	39,129,352	24,172,957,000
Convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount (in shares)	22,687,625	22,687,625,000
Convertible Debt | 2020 Convertible Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount (in shares)	10,741,406	0
Convertible Debt | 2020 Gilead Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount (in shares)	210,752	0
Employee stock purchase plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount (in shares)	3,359,622	389,086,000
Early exercised stock options subject to future vesting
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount (in shares)	1,858,264	854,123,000
Restricted stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount (in shares)	271,683	242,123,000

Commitment and Contingencies -

Commitment and Contingencies - Narrative (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	9 Months Ended
	Jul. 31, 2020	Sep. 30, 2020	Sep. 30, 2020
2020 Gilead Notes | Convertible Notes
Schedule Of Asset Acquisition [Line Items]
Interest rate		6.00%	6.00%
Repayment of debt, payments to be made			$ 6
2020 Notes	$ 3.6	$ 5.5	5.5
Gain (loss) in fair value		$ 1.9	$ 1.9
Asset Purchase Agreement
Schedule Of Asset Acquisition [Line Items]
Sales milestone payment	29
Regulatory milestone payment	51.3
Aggregate sales milestone payment	115
Gilead Asset Purchase
Schedule Of Asset Acquisition [Line Items]
Upfront cash payment	3
Convertible promissory note issued	3
Payments for reimbursements	$ 0.7

Commitment and Contingencies _2

Commitment and Contingencies - Key Valuation Assumptions (Details)	Sep. 30, 2020	Jul. 31, 2020
Remaining term (years) | Fundraise scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	1.78	2
Remaining term (years) | IPO cash payout scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	0.08	0.30
Remaining term (years) | IPO conversion scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	0.08	0.30
Remaining term (years) | Liquidation cash payout scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	1.79	2
Remaining term (years) | Liquidation conversion scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	1.79	2
Discount rate | Fundraise scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	0.242	0.242
Discount rate | IPO cash payout scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	0.242	0.242
Discount rate | IPO conversion scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	0.242	0.242
Discount rate | Liquidation cash payout scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	0.242	0.242
Discount rate | Liquidation conversion scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	0.242	0.242
Weighting | Fundraise scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	0.015	0.186
Weighting | IPO cash payout scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	0.950	0.129
Weighting | IPO conversion scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	0	0
Weighting | Liquidation cash payout scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	0.020	0.500
Weighting | Liquidation conversion scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	0.015	0.186

Leases - Additional Information

Leases - Additional Information (Details)	Jul. 01, 2020USD ($)ft²	Mar. 01, 2020USD ($)ft²	Aug. 01, 2018USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2019USD ($)	Dec. 31, 2019USD ($)		Jul. 31, 2018ft²	Mar. 31, 2018
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets				$ 29,355,000		$ 29,355,000		$ 473,000	[1]
Lease liabilities				30,784,000		30,784,000		496,000
Lease payments				700,000	$ 100,000	1,300,000	$ 200,000
R&D Equipment
Lessee, Lease, Description [Line Items]
Finance lease, term											3 years
21 Erie Street, Cambridge, Massachusetts
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets				1,600,000		1,600,000
Lease liabilities				$ 1,000,000		$ 1,000,000
Remaining lease term				8 months		8 months
Incremental borrowing rate				12.71%		12.71%
301 Binney Street, Cambridge, Massachusetts
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets				$ 25,700,000		$ 25,700,000
Lease liabilities				$ 27,700,000		$ 27,700,000
Remaining lease term				10 years 6 months		10 years 6 months
Incremental borrowing rate				8.50%		8.50%
Lease term		11 years
Facility size | ft²		40,514
Monthly base rent		$ 300,000
Escalating rent payments, percent		3.00%
Letter of credit				$ 2,000,000		$ 2,000,000
Tenant improvement allowances		$ 3,700,000
San Mateo, California
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets				1,200,000		1,200,000		500,000
Lease liabilities				1,200,000		1,200,000		$ 500,000
Lease term			37 months
Facility size | ft²										4,661
Escalating rent payments, percent			3.00%
Security deposit			$ 27,800
Initial annual base rent			$ 26,100
San Mateo, California, Adjacent Space
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets				900,000		900,000
Lease liabilities				$ 900,000		$ 900,000
Remaining lease term				4 years 8 months 12 days		4 years 8 months 12 days
Incremental borrowing rate				12.07%		12.07%
Facility size | ft²	3,414
Escalating rent payments, percent	3.00%
Security deposit	$ 24,800
Initial annual base rent	$ 19,200
[1]	The balance sheet as of December 31, 2019 is derived from the audited financial statements as of that date.

Leases - Summary of Lease Costs

Leases - Summary of Lease Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Lessee, Lease, Description [Line Items]
Finance lease cost	$ 5	$ 6	$ 16	$ 18
Interest income (expense), net	0	1	1	3
Operating lease cost	1,785	78	3,733	233
Research and development
Lessee, Lease, Description [Line Items]
Finance lease cost	5	5	15	15
Operating lease cost	1,397	0	2,843	0
General and administrative expenses
Lessee, Lease, Description [Line Items]
Operating lease cost	$ 388	$ 78	$ 890	$ 233

Leases - Supplemental Balance S

Leases - Supplemental Balance Sheet Information - Finance Lease (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets:
Property and equipment, net	$ 88	$ 103
Liabilities:
Current portion of finance lease liability	14	32
Other noncurrent liabilities	0	5
Total finance lease liabilities	$ 14	$ 37

Leases - Supplemental Balance_2

Leases - Supplemental Balance Sheet Information - Operating Leases (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets:
Operating lease assets	$ 29,355	$ 473	[1]
Liabilities:
Current portion of operating lease liabilities	1,300	285	[1]
Noncurrent operating lease liabilities	29,484	211	[1]
Total operating lease liabilities	$ 30,784	$ 496
[1]	The balance sheet as of December 31, 2019 is derived from the audited financial statements as of that date.

Leases - Schedule of Maturities

Leases - Schedule of Maturities on Lease Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Leases [Abstract]
Remaining three months of 2020	$ 1,329
2021	5,770
2022	4,789
2023	4,933
2024 and thereafter	35,915
Total undiscounted lease payments	52,736
Less: Present value adjustment	(18,238)
Less: Tenant improvement allowance	(3,714)
Present value of operating lease liabilities	$ 30,784	$ 496

2020 Notes - Narrative (Details

2020 Notes - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended	9 Months Ended
	Oct. 31, 2020	Aug. 31, 2020	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019
Debt Instrument [Line Items]
Net cash proceeds				$ 151,275	$ 0
Total debt issuance costs expensed and recognized				3,889	$ 0
Common Stock | Subsequent Event
Debt Instrument [Line Items]
Shares issued upon conversion of convertible preferred stock (in shares)	22,687,625
IPO
Debt Instrument [Line Items]
Settlement price, percentage		85.00%
Valuation of company		$ 500,000
IPO | Common Stock | Subsequent Event
Debt Instrument [Line Items]
Shares issued upon conversion of convertible preferred stock (in shares)	22,687,625
2020 Convertible Notes | IPO | Common Stock | Subsequent Event
Debt Instrument [Line Items]
Shares issued upon conversion of convertible preferred stock (in shares)	9,610,713
Convertible Notes | Convertible Notes Payable (2020 Notes)
Debt Instrument [Line Items]
Debt instrument, face amount		155,200
Net cash proceeds		151,300
2020 Notes		155,200	$ 168,500	168,500
Change in fair value of 2020 Notes			$ 13,300	$ 13,300
Total debt issuance costs expensed and recognized		3,900
Reimbursement for legal costs		$ 50

2020 Notes - Key Value Assumpti

2020 Notes - Key Value Assumptions (Details)	Sep. 30, 2020
Remaining term (years) | IPO Scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	0.08
Remaining term (years) | Series B Scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	1.39
Remaining term (years) | Financing Scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	1.39
Discount rate | IPO Scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	0.242
Discount rate | Series B Scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	0.242
Discount rate | Financing Scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	0.242
Weighting | IPO Scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	0.950
Weighting | Series B Scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	0.025
Weighting | Financing Scenario
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Key valuation assumptions	0.025

Related Party Disclosures (Deta

Related Party Disclosures (Details) - Affiliated Entity - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Related Party Transaction [Line Items]
Reimbursable expenses to related parties	$ 15,000	$ 7,400	$ 24,100	$ 44,900
Payables to related parties	92,500		$ 92,500		$ 75,000
Two River Consulting, LLC
Related Party Transaction [Line Items]
Services agreement term			3 years
Expenses from transaction with related party	300,000	200,000	$ 800,000	600,000
Bellco Capital, LLC
Related Party Transaction [Line Items]
Expenses from transaction with related party	$ 6,300	$ 6,300	$ 18,900	$ 18,900

Subsequent Events (Details)

Subsequent Events (Details) $ / shares in Units, $ in Millions	Oct. 02, 2020shares	Nov. 18, 2020USD ($)	Oct. 14, 2020$ / sharesshares	Oct. 08, 2020shares	Sep. 30, 2020shares	Dec. 31, 2019shares	Jul. 01, 2019shares
Subsequent Event [Line Items]
Common stock, shares authorized (in shares)					40,000,000	40,000,000	40,000,000
Cambridge, Massachusetts
Subsequent Event [Line Items]
Cost for early lease termination | $		$ 0.1
Convertible Notes | 2020 Convertible Notes
Subsequent Event [Line Items]
Gain (loss) on settlement of convertible notes | $		(14.1)
Convertible Notes | 2020 Gilead Notes
Subsequent Event [Line Items]
Gain (loss) on settlement of convertible notes | $		$ 1.9
Subsequent Event
Subsequent Event [Line Items]
Preferred stock, shares authorized (in shares)			10,000,000
Preferred stock, par value (in dollars per share) | $ / shares			$ 0.001
Common stock, shares authorized (in shares)	200,000,000
Subsequent Event | Common Stock
Subsequent Event [Line Items]
Forward stock split ratio	1.055
Subsequent Event | The 2020 Plan | Stock options
Subsequent Event [Line Items]
Number of shares that may have been issued (in shares)				11,938,152
Subsequent Event | The ESPP | Employee stock purchase plan
Subsequent Event [Line Items]
Number of shares that may have been issued (in shares)				688,000