Generation Bio (GBIO) S-1 4 Jan 21 IPO registration Financial data

Cover Page

Cover Page	9 Months Ended
	Sep. 30, 2020
Cover [Abstract]
Document Type	S-1
Amendment Flag	false
Entity Central Index Key	0001733294
Entity Registrant Name	GENERATION BIO CO.
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 58,605	$ 15,076	$ 7,305
Marketable securities	221,761		67,565
Tenant receivable		448	1,288
Prepaid expenses and other current assets	5,447	2,577	714
Restricted cash		55	54
Total current assets	285,813	18,156	76,926
Property and equipment, net	22,888	21,845	4,486
Restricted cash, noncurrent	2,052	2,052	2,107
Deferred offering costs		87
Other long-term assets	169
Total assets	310,922	42,140	83,519
Current liabilities:
Accounts payable	418	2,251	1,681
Accrued expenses and other current liabilities	8,745	6,907	4,874
Total current liabilities	9,163	9,158	6,555
Deferred rent, net of current portion	15,220	15,981	2,962
Total liabilities	24,383	25,139	9,517
Commitments and contingencies (Note 9)
Convertible preferred stock (Series A, B and C), $0.0001 par value; 26,425,664 shares and no shares authorized at December 31, 2019 and September 30, 2020, respectively; 26,425,664 shares and no shares issued and outstanding at December 31, 2019 and September 30, 2020, respectively		115,593	115,593
Stockholders' equity (deficit):
Preferred stock, $0.0001 par value; no shares and 5,000,000 shares authorized at December 31, 2019 and September 30, 2020, respectively; no shares issued or outstanding
Common stock, $0.0001 par value; 46,750,000 and 150,000,000 shares authorized at December 31, 2019 and September 30, 2020, respectively; 6,976,077 and 46,516,475 shares issued at December 31, 2019 and September 30, 2020, respectively; 5,270,889 and 45,559,757 shares outstanding at December 31, 2019 and September 30, 2020, respectively	5	1	1
Additional paid-in capital	451,257	9,859	5,552
Accumulated other comprehensive income	26		(9)
Accumulated deficit	(164,749)	(108,452)	(47,135)
Total stockholders' equity (deficit)	286,539	(98,592)	(41,591)
Total liabilities, convertible preferred stock and stockholders' equity (deficit)	$ 310,922	$ 42,140	$ 83,519

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Convertible preferred stock, par value per share	$ 0.0001	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	0	26,425,664	26,425,664
Convertible preferred stock, shares issued	0	26,425,664	26,425,664
Convertible preferred stock, shares outstanding	0	26,425,664	26,425,664
Preferred stock, par value per share	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value per share	$ 0.0001	$ 0.0001	$ 0.0001
Common stock, shares authorized	150,000,000	46,750,000	46,750,000
Common stock, shares issued	46,516,475	6,976,077	6,874,424
Common stock, shares outstanding	45,559,757	5,270,889	4,054,475
Liquidation preference		$ 115,822	$ 115,822

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Revenues						$ 36
Operating expenses:
Research and development	$ 15,308	$ 13,447	$ 42,158	$ 37,403	$ 50,134	28,152
General and administrative	5,661	2,886	14,611	8,741	12,168	9,178
Total operating expenses	20,969	16,333	56,769	46,144	62,302	37,330
Loss from operations	(20,969)	(16,333)	(56,769)	(46,144)	(62,302)	(37,294)
Other income (expense):
Interest income and other income (expense), net	120	205	472	890	985	1,491
Net loss and net loss attributable to common stockholders	$ (20,849)	$ (16,128)	$ (56,297)	$ (45,254)	$ (61,317)	$ (35,803)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.46)	$ (3.31)	$ (2.68)	$ (9.90)	$ (12.96)	$ (10.69)
Weighted average common shares outstanding, basic and diluted	45,468,838	4,866,830	21,011,439	4,571,641	4,731,519	3,350,537
Comprehensive loss:
Net loss	$ (20,849)	$ (16,128)	$ (56,297)	$ (45,254)	$ (61,317)	$ (35,803)
Other comprehensive income (loss):
Unrealized gains (losses) on marketable securities	30	(9)	26	11	9	(9)
Comprehensive loss	$ (20,819)	$ (16,137)	$ (56,271)	$ (45,243)	$ (61,308)	$ (35,812)

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series C Convertible Preferred Stock [Member]	Series B Convertible Preferred Stock [Member]	Series A, B and C Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance, convertible preferred stock at Dec. 31, 2017				$ 15,404
Beginning balance, convertible preferred stock, shares at Dec. 31, 2017				15,451,020
Beginning balance at Dec. 31, 2017	$ (9,267)				$ 1	$ 2,064		$ (11,332)
Beginning balance, shares at Dec. 31, 2017					2,917,665
Issuance of Series C convertible preferred stock, net of issuance costs of $2,640, shares			10,974,644
Issuance of Series C convertible preferred stock, net of issuance costs of $2,640			$ 100,189
Stock-based compensation expense	3,488					3,488
Unrealized gains (losses) on marketable securities							$ (9)	(9)
Net loss	(35,803)							(35,803)
Ending balance, convertible preferred stock at Dec. 31, 2018	$ 115,593			$ 115,593
Ending balance, convertible preferred stock, shares at Dec. 31, 2018	26,425,664			26,425,664
Ending balance at Dec. 31, 2018	$ (41,591)				$ 1	5,552	(9)	(47,135)
Ending balance, shares at Dec. 31, 2018					4,054,475
Vesting of restricted common stock, shares					1,136,810
Issuance of common stock for license,shares					125,677
Stock-based compensation expense	3,122					3,122
Unrealized gains (losses) on marketable securities	11						11
Net loss	(45,254)							(45,254)
Ending balance, convertible preferred stock at Sep. 30, 2019				$ 115,593
Ending balance, convertible preferred stock, shares at Sep. 30, 2019				26,425,664
Ending balance at Sep. 30, 2019	(83,703)				$ 1	8,683	2	(92,389)
Ending balance, shares at Sep. 30, 2019					4,989,446
Issuance of common stock upon exercise of stock options	9					9
Issuance of common stock upon exercise of stock options, shares					14,861
Vesting of restricted common stock, shares					794,433
Beginning balance, convertible preferred stock at Dec. 31, 2018	$ 115,593			$ 115,593
Beginning balance, convertible preferred stock, shares at Dec. 31, 2018	26,425,664			26,425,664
Beginning balance at Dec. 31, 2018	$ (41,591)				$ 1	5,552	(9)	(47,135)
Beginning balance, shares at Dec. 31, 2018					4,054,475
Issuance of common stock for license,shares					125,677
Stock-based compensation expense	4,207					4,207
Unrealized gains (losses) on marketable securities	9						9
Net loss	(61,317)							(61,317)
Ending balance, convertible preferred stock at Dec. 31, 2019	$ 115,593			$ 115,593
Ending balance, convertible preferred stock, shares at Dec. 31, 2019	26,425,664		10,974,644	26,425,664
Ending balance at Dec. 31, 2019	$ (98,592)				$ 1	9,859		(108,452)
Ending balance, shares at Dec. 31, 2019					5,270,889
Issuance of common stock upon exercise of stock options	$ 100					100
Issuance of common stock upon exercise of stock options, shares	34,740				34,740
Vesting of restricted common stock, shares					1,055,997
Beginning balance, convertible preferred stock at Jun. 30, 2019				$ 115,593
Beginning balance, convertible preferred stock, shares at Jun. 30, 2019				26,425,664
Beginning balance at Jun. 30, 2019	$ (68,646)				$ 1	7,603	11	(76,261)
Beginning balance, shares at Jun. 30, 2019					4,615,383
Issuance of common stock for license,shares					125,677
Stock-based compensation expense	1,080					1,080
Unrealized gains (losses) on marketable securities	(9)						(9)
Net loss	(16,128)							(16,128)
Ending balance, convertible preferred stock at Sep. 30, 2019				$ 115,593
Ending balance, convertible preferred stock, shares at Sep. 30, 2019				26,425,664
Ending balance at Sep. 30, 2019	(83,703)				$ 1	8,683	2	(92,389)
Ending balance, shares at Sep. 30, 2019					4,989,446
Vesting of restricted common stock, shares					248,386
Beginning balance, convertible preferred stock at Dec. 31, 2019	$ 115,593			$ 115,593
Beginning balance, convertible preferred stock, shares at Dec. 31, 2019	26,425,664		10,974,644	26,425,664
Beginning balance at Dec. 31, 2019	$ (98,592)				$ 1	9,859		(108,452)
Beginning balance, shares at Dec. 31, 2019					5,270,889
Issuance of Series C convertible preferred stock, net of issuance costs of $2,640, shares		19,936,296
Issuance of Series C convertible preferred stock, net of issuance costs of $2,640		$ 108,832
Conversion of convertible preferred stock into common stock upon initial public offering, convertible preferred stock				$ (224,425)
Conversion of convertible preferred stock into common stock upon initial public offering, convertible preferred stock, shares				(46,361,960)
Conversion of convertible preferred stock into common stock upon initial public offering	224,425				$ 3	224,422
Conversion of convertible preferred stock into common stock upon initial public offering, shares					27,094,085
Issuance of common stock upon initial public offering, net of issuance costs of $3,185	210,715				$ 1	210,714
Issuance of common stock upon initial public offering, net of issuance costs of $3,185, shares					12,105,263
Stock-based compensation expense	5,033					5,033
Unrealized gains (losses) on marketable securities	26						26
Net loss	$ (56,297)							(56,297)
Ending balance, convertible preferred stock, shares at Sep. 30, 2020	0
Ending balance at Sep. 30, 2020	$ 286,539				$ 5	451,257	26	(164,749)
Ending balance, shares at Sep. 30, 2020					45,559,757
Issuance of common stock upon exercise of stock options	1,229					1,229
Issuance of common stock upon exercise of stock options, shares					319,642
Vesting of restricted common stock, shares					769,878
Beginning balance at Jun. 30, 2020	304,153				$ 5	448,052	(4)	(143,900)
Beginning balance, shares at Jun. 30, 2020					45,028,024
Stock-based compensation expense	2,161					2,161
Unrealized gains (losses) on marketable securities	30						30
Net loss	$ (20,849)							(20,849)
Ending balance, convertible preferred stock, shares at Sep. 30, 2020	0
Ending balance at Sep. 30, 2020	$ 286,539				$ 5	451,257	$ 26	$ (164,749)
Ending balance, shares at Sep. 30, 2020					45,559,757
Issuance of common stock upon exercise of stock options	$ 1,044					$ 1,044
Issuance of common stock upon exercise of stock options, shares					279,379
Vesting of restricted common stock, shares					252,354

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) $ in Thousands	9 Months Ended
	Sep. 30, 2020USD ($)
Common Stock [Member] | IPO [Member]
Stock issuance of costs	$ 3,185
Series C Convertible Preferred Stock [Member]
Stock issuance of costs	$ 2,640

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net loss	$ (56,297)	$ (45,254)	$ (61,317)	$ (35,803)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	5,033	3,122	4,207	3,488
Depreciation and amortization expense	2,441	1,185	1,864	347
Amortization (accretion) of premium (discount) on marketable securities	88	(335)	(317)	(834)
Gain on sale of property and equipment			(10)
Changes in operating assets and liabilities:
Tenant receivable	448	(1,297)	840	(1,288)
Prepaid expenses and other current assets	(2,870)	(1,078)	(1,863)	(498)
Other noncurrent assets	(169)			17
Accounts payable	(1,321)	(132)	761	223
Accrued expenses and other current liabilities	1,631	2,284	2,470	3,263
Deferred rent	(761)	12,966	13,019	2,966
Net cash used in operating activities	(51,777)	(28,539)	(40,346)	(28,119)
Cash flows from investing activities:
Purchases of property and equipment	(3,724)	(18,828)	(19,986)	(3,021)
Proceeds from sale of property and equipment			80
Purchases of marketable securities	(221,823)	(20,789)	(20,789)	(109,940)
Sales and maturities of marketable securities		82,680	88,680	43,200
Net cash provided by (used in) investing activities	(225,547)	43,063	47,985	(69,761)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs incurred and paid in current period	108,854
Proceeds from initial public offering of common stock, net of underwriting discounts and commissions	213,900			100,189
Proceeds from exercise of stock options	1,229	9	100	0
Payment of initial public offering costs	(3,185)
Payments of deferred offering costs			(22)
Net cash provided by financing activities	320,798	9	78	100,189
Net increase in cash, cash equivalents and restricted cash	43,474	14,533	7,717	2,309
Cash, cash equivalents and restricted cash at beginning of period	17,183	9,466	9,466	7,157
Cash, cash equivalents and restricted cash at end of period	60,657	23,999	17,183	9,466
Supplemental disclosure of noncash investing and financing information:
Conversion of convertible preferred stock to common stock	224,425
Purchases of property and equipment included in accounts payable and accrued expenses	$ 404	$ 895	644	$ 1,337
Preferred stock issuance costs included in accrued expenses			$ 65

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation The Company was incorporated on October 21, 2016 non-viral non-viral The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations, the ability to establish clinical- and commercial-scale manufacturing processes, the impact of the COVID-19 On June 16, 2020, the Company completed its initial public offering (“IPO”) pursuant to which it issued and sold 12,105,263 shares of its common stock, including 1,578,947 shares pursuant to the full exercise of the underwriters’ option to purchase additional shares, at a public offering price of $19.00 per share, resulting in net proceeds of $210.7 million, after deducting underwriting discounts and commissions and other offering expenses. Upon the closing of the IPO, all of the Company’s outstanding convertible preferred stock automatically converted into shares of common stock. The accompanying consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. Since inception, the Company has funded its operations with proceeds from the sales of instruments convertible into convertible preferred stock (which converted into convertible preferred stock in 2017), the sale of convertible preferred stock, and most recently, with proceeds from the sale of common stock in the IPO completed in June 2020. The Company has incurred recurring losses, including net losses of $61.3 million for the year ended December 31, 2019 and $56.3 million for the nine months ended September 30, 2020. As of September 30, 2020, the Company had an accumulated deficit of $164.7 million. The Company expects to continue to generate operating losses in the foreseeable future. As of November 10, 2020, the issuance date of these interim condensed consolidated financial statements, the Company expects that its cash, cash equivalents and marketable securities will be sufficient to fund its operating expenses and capital expenditure requirements for at least 12 months. The Company will need to obtain additional funding through public or private equity offerings, debt financings, collaborations, strategic alliances and/or licensing arrangements. 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 collaborations or strategic alliances or licensing arrangements. The terms of any financing may adversely affect the holdings or the rights of the Company’s stockholders. Arrangements with collaborators or others may require the Company to relinquish rights to certain of its technologies or programs. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate some or all of its research and development programs, pipeline expansion or commercialization efforts, which could adversely affect its business prospects. Although management will continue to pursue these plans, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations when needed or at all. The accompanying consolidated financial statements reflect the operations of the Company and the Company’s wholly owned subsidiary, Generation Bio Securities Corporation. Intercompany balances and transactions have been eliminated in consolidation. The accompanying consolidated financial statements have been prepared in conformity with generally accepted accounting principles (“GAAP”) in the United States of America. Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”).	1. Nature of the business and basis of presentation The Company was incorporated on October 21, 2016 as Torus Therapeutics, Inc. and subsequently changed its name to Generation Bio Co. (the “Company” or “Generation Bio”). The Company is an innovative genetic medicines company creating a new class of gene therapy utilizing its proprietary non-viral non-viral The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations, the ability to establish clinical- and commercial-scale manufacturing processes and the ability to secure additional capital to fund operations. Programs currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization of a product. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. The accompanying consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. Through December 31, 2019, the Company has funded its operations with the proceeds from instruments convertible into convertible preferred stock (which converted into convertible preferred stock in 2017) and the sale of convertible preferred stock. Since inception, the Company has incurred recurring losses, including net losses of $61.3 million for the year ended December 31, 2019. As of December 31, 2019, the Company had an accumulated deficit of $108.5 million. The Company expects to continue to generate operating losses in the foreseeable future. As of April 9, 2020, the issuance date of the annual consolidated financial statements for the year ended December 31, 2019, the Company expected that its cash and cash equivalents, including gross proceeds of $111.5 million it received in January 2020 from the sale of Series C convertible preferred stock (see Note 13), would be sufficient to fund its operating expenses and capital expenditure requirements for at least 12 months from the issuance date of the annual consolidated financial statements. The Company is seeking to complete an initial public offering (“IPO”) of its common stock. Upon the completion of a qualified public offering on specified terms, the Company’s outstanding convertible preferred stock will automatically convert into shares of common stock (see Note 6). The Company will need to obtain additional funding through public or private equity offerings, debt financings, collaborations, strategic alliances and/or licensing arrangements. 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 collaborative or strategic alliances or licensing arrangements. The terms of any financing may adversely affect the holdings or the rights of the Company’s stockholders. Arrangements with collaborators or others may require the Company to relinquish rights to certain of its technologies or programs. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate some or all of its research and development programs, pipeline expansion or commercialization efforts, which could adversely affect its business prospects. Although management will continue to pursue these plans, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations when needed or at all. The accompanying consolidated financial statements reflect the operations of the Company and the Company’s wholly owned subsidiary, Generation Bio Securities Corporation. Intercompany balances and transactions have been eliminated in consolidation. The accompanying consolidated financial statements have been prepared in conformity with generally accepted accounting principles (“GAAP”) in the United States of America. Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”).

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies 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 contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these condensed consolidated financial statements include, but are not limited to, the accrual of research and development expenses and stock-based compensation expense. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results may differ from those estimates or assumptions. Unaudited interim financial information The balance sheet at December 31, 2019 was derived from audited financial statements but does not include all disclosures required by GAAP. The accompanying unaudited financial statements as of September 30, 2020 and for the three and nine months ended September 30, 2019 and 2020 have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. These financial statements should be read in conjunction with the Company’s audited financial statements included elsewhere in this prospectus. In the opinion of management, all adjustments, consisting only of normal recurring adjustments necessary for a fair presentation of the Company’s financial position as of September 30, 2020, the results of operations for the three and nine months ended September 30, 2019 and 2020, and cash flows for the nine months ended September 30, 2019 and 2020 have been made. The results of operations for the three and nine months ended September 30, 2020 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2020 or any other period. Concentrations of credit risk and of significant suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents, and marketable securities. The Company believes it is not exposed to significant credit risk due to the financial strength of the depository institutions in which the cash and cash equivalents are held. The Company maintains its cash equivalents in money market funds that invest in U.S. treasury securities. The Company’s marketable securities as of September 30, 2020 consisted of U.S government treasury securities, U.S. government agency bonds, commercial paper and corporate debt securities. The Company has adopted an investment policy that limits the amounts the Company may invest in the securities of single issuer with the exclusion of the U.S. government. The Company has not experienced any credit losses and does not believe that it is subject to significant credit risk. The Company is dependent on a small number of third-party suppliers for its drug substance and drug product. In particular, the Company relies, and expects to continue to rely, on third-party suppliers for certain materials and components required for the production of any product candidates it may develop for its programs. These programs could be adversely affected by a significant interruption in the supply process. Fair value measurements Certain assets and liabilities of the Company are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) 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. Financial assets and financial liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents and marketable securities are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities. Recently adopted accounting pronouncements In August 2018, the FASB issued ASU 2018-13, 2018-13 2018-03 Recently issued accounting pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies that the Company adopts as of the specified effective date. The Company qualifies as an “emerging growth company” as defined in the Jumpstart Our Business Startups Act of 2012 and has elected not to “opt out” of the extended transition related to complying with new or revised accounting standards, which means that when a standard is issued or revised and it has different application dates for public and nonpublic companies, the Company can adopt the new or revised standard at the time nonpublic companies adopt the new or revised standard and can do so until such time that the Company either (i) irrevocably elects to “opt out” of such extended transition period or (ii) no longer qualifies as an emerging growth company. In February 2016, the FASB issued ASU 2016-02, 2018-11, 2019-10, 2020-05, one-year 2016-02 right-of-use In June 2016, the FASB issued ASU 2016-13, available-for-sale. 2019-10,	2. Summary of significant accounting policies 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 contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses and stock-based compensation expense. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results may differ from those estimates or assumptions. Concentrations of credit risk and of significant suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. The Company has not experienced any other-than-temporary losses with respect to its cash and cash equivalents and does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on a small number of third-party suppliers for its drug substance and drug product. In particular, the Company relies, and expects to continue to rely, on third-party suppliers for certain materials and components required for the production of any product candidates it may develop for its programs. These programs could be adversely affected by a significant interruption in the supply process. Deferred offering costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity paid-in Cash equivalents The Company considers all highly liquid investments with a remaining maturity when purchased of three months or less to be cash equivalents. Restricted cash Amounts included in restricted cash represent amounts pledged as collateral for letters of credit required for security deposits on the Company’s leased facilities. These amounts are classified as restricted cash (current and noncurrent) on the Company’s consolidated balance sheets. Marketable securities The Company’s marketable securities, which consisted of debt securities as of December 31, 2018, are classified as available-for-sale and on available-for-sale debt The Company evaluates its marketable securities with unrealized losses for other-than-temporary impairment. When assessing marketable securities for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, how long the market value of the investment has been less than its original cost, the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary,” the Company reduces the investment to fair value through a charge to the statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented. Fair value measurements Certain assets and liabilities of the Company are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) 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. Financial assets and financial liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents and marketable securities are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s tenant receivable, accounts payable and accrued expenses approximate their fair values due to the short-term nature of these assets and liabilities. Deferred rent Payment escalations, rent holidays and other lease incentives that may be included in lease agreements are accrued or deferred as appropriate such that rent expense for each lease is recognized on a straight-line basis over the respective lease term. Adjustments for such items are recorded as deferred rent and amortized over the respective lease terms. Property and equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset as follows: Estimated useful life Laboratory equipment 5 years Computer equipment and software 3 years Furniture and fixtures 5 years Leasehold improvements Shorter of remaining life of lease or useful life Costs for capital assets not yet placed into service are capitalized as construction-in-progress Impairment of long-lived assets Long-lived assets consist of property and equipment. The Company evaluates the recoverability of its long-lived assets when circumstances indicate that an event of impairment may have occurred. The Company recognizes an impairment loss only if the carrying amount of a long-lived asset is not recoverable based on its undiscounted future cash flows. Impairment is measured based on the difference between the carrying value of the related assets and the fair value of such assets. The Company did not record any impairment losses on long-lived assets during the years ended December 31, 2018 or 2019. Classification and accretion of convertible preferred stock The Company’s convertible preferred stock is classified outside of stockholders’ equity (deficit) on the consolidated balance sheets because the holders of such shares have redemption rights in the event of a deemed liquidation that, in certain situations, is not solely within the control of the Company and would require the redemption of the then-outstanding convertible preferred stock. A deemed liquidation includes Liquidation Events as that term is defined in Note 6. The Company’s Series A and Series B convertible preferred stock are not redeemable, except in the event of a deemed liquidation (see Note 6). Because the occurrence of a deemed liquidation event is not currently probable, the carrying values of the convertible preferred stock are not being accreted to their redemption values. If a deemed liquidation event became probable, the carrying values of the convertible preferred stock would be accreted to redemption values. Segment information The Company has determined that its chief executive officer is the chief operating decision maker (“CODM”). The CODM reviews financial information presented on a consolidated basis. Resource allocation decisions are made by the CODM based on consolidated results. There are no segment managers who are held accountable by the CODM for operations, operating results, and planning for levels or components below the consolidated unit level. As such, the Company has concluded that it operates as one segment. All long-lived assets are located in the United States. Revenue recognition To date, the Company has derived its revenue by providing services to pharmaceutical and life sciences companies. On January 1, 2019, the Company adopted ASU 2014-09, The Company only applies the five-step model to contracts when collectability of the consideration to which the Company is entitled in exchange for the goods or services it transfers to the customer is determined to be probable. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is that the period between payment by the customer and the transfer of the promised goods or services to the customer will be one year or less. The Company recognizes revenue when it satisfies its performance obligations by delivering the services to its customers in an amount that reflects the consideration to which it expects to be entitled in exchange for those services. Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue on the accompanying balance sheets. Amounts expected to be recognized as revenue within 12 months of the balance sheet date are classified as current deferred revenue. The Company had no deferred revenue as of December 31, 2019. The Company expenses incremental costs of obtaining a contract as and when incurred if the expected amortization period of the asset that would have been recognized is one year or less or the amount is immaterial. As of December 31, 2019, the Company had not capitalized any costs to obtain contracts. Research and development costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries and bonuses, stock-based compensation, employee benefits, facilities costs, laboratory supplies, depreciation and amortization, manufacturing expenses and external costs of vendors engaged to conduct preclinical development activities as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology are expensed as research and development expenses in the period in which they are incurred. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed. Research and manufacturing contract costs and accruals The Company has entered into various research and development and manufacturing contracts. These agreements are generally cancelable, and related payments are recorded as the corresponding expenses are incurred. The Company records accruals for estimated ongoing costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the research studies and manufacturing activities, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs. Patent costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses. Stock-based compensation The Company measures stock options with service-based vesting or performance-based vesting granted to employees, non-employees Compensation expense for awards with service-based vesting is generally recognized over the vesting period of the award using the straight-line method to record the expense. The Company uses the graded-vesting method to record the expense of awards with both service-based and performance-based vesting conditions, commencing once achievement of the performance condition becomes probable. The Company accounts for forfeitures of share-based awards as they occur. The Company classifies stock-based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified. Prior to the adoption of ASU 2018-07 non-employees non-employees. 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 years ended December 31, 2018 and 2019, the Company’s only element of other comprehensive loss was unrealized gains (losses) on marketable securities. Income taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process deemed more-likely-than-not to Net loss per share The Company follows the two-class two-class two-class Basic net income (loss) per share attributable to common stockholders is computed by dividing net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding for the period. Diluted net income (loss) per share attributable to common stockholders is computed by adjusting net income (loss) attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common stockholders is computed by dividing the diluted net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares. The Company’s participating securities contractually entitle the holders of such shares to participate in dividends but do not contractually require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss, such losses are not allocated to such participating securities. In periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. The Company reported a net loss attributable to common stockholders for the years ended December 31, 2018 and 2019. Recently adopted accounting pronouncements In May 2014, the FASB issued ASU 2014-09, 2014-09 2014-09 In June 2018, the FASB issued ASU 2018-07, ASU 2018-07 was For non-public entities, ASU 2018-07 is ASU 2014-09. The adopted ASU 2018-07 on In August 2018, the FASB issued ASU 2018-13, 2018-13 2018-03 Recently issued accounting pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies that the Company adopts as of the specified effective date. The Company qualifies as an “emerging growth company” as defined in the Jumpstart Our Business Startups Act of 2012 and has elected not to “opt out” of the extended transition related to complying with new or revised accounting standards, which means that when a standard is issued or revised and it has different application dates for public and nonpublic companies, the Company can adopt the new or revised standard at the time nonpublic companies adopt the new or revised standard and can do so until such time that the Company either (i) irrevocably elects to “opt out” of such extended transition period or (ii) no longer qualifies as an emerging growth company. In February 2016, the FASB issued ASU 2016-02, 2018-11, 2019-10, entities to annual reporting periods beginning after December 15, 2020, and interim periods within fiscal years beginning after December 15, 2021. Early adoption continues to be allowed. The Company will adopt ASU 2016-02 right-of-use In June 2016, the FASB issued ASU 2016-13, available-for-sale. 2019-10,

Fair Value of Financial Assets

Fair Value of Financial Assets	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets	3. Fair Value of Financial Assets The Company did not have marketable securities at December 31, 2019. As of September 30, 2020, marketable securities by security type consisted of the following: Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value (in thousands) U.S. treasury securities $ 132,184 $ 16 $ — $ 132,200 U.S. government agency bonds 11,995 2 — 11,997 Commercial paper 59,891 — — 59,891 Corporate debt securities 17,665 8 — 17,673 Totals $ 221,735 $ 26 $ — $ 221,761 As of September 30, 2020, marketable securities consisted of investments that mature within one year. The following tables present the Company’s assets that are measured at fair value on a recurring basis and indicate the level within the fair value hierarchy of the valuation techniques the Company utilized to determine such fair value, which is described further within Note 2, Summary of Significant Accounting Policies: Fair Value Measurements at Level 1 Level 2 Level 3 Total (in thousands) Cash equivalents: Money market funds $ 15,076 $ — $ — $ 15,076 Totals $ 15,076 $ — $ — $ 15,076 Fair Value Measurements at September 30, Level 1 Level 2 Level 3 Total (in thousands) Cash equivalents: Money market funds $ 57,394 $ — $ — $ 57,394 Marketable securities: U.S. treasury securities — 132,200 — 132,200 U.S. government agency bonds — 11,997 — 11,997 Commercial paper — 59,891 — 59,891 Corporate debt securities — 17,673 — 17,673 Totals $ 57,394 $ 221,761 $ — $ 279,155 Money market funds were valued based on quoted market prices, which represent a Level 1 measurement within the fair value hierarchy. The Company’s marketable securities, which consisted of U.S. treasury securities, U.S. government agency bonds, commercial paper and corporate debt securities were valued using quoted prices in active markets for similar securities, which represent a Level 2 measurement within the fair value hierarchy.	3. Marketable securities and fair value measurements Marketable securities by security type consisted of the following: December 31, 2018 (in thousands) Amortized Gross Gross Fair U.S. treasury bills and notes $ 67,574 $ — $ (9 ) $ 67,565 $ 67,574 $ — $ (9 ) $ 67,565 The Company did not have marketable securities as of December 31, 2019. The following tables present the Company’s assets that are measured at fair value on a recurring basis and indicate the level within the fair value hierarchy of the valuation techniques the Company utilized to determine such fair value, which is described further within Note 2, Fair value measurements at December 31, (in thousands) Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 7,244 $ — $ — $ 7,244 Marketable securities U.S. treasury bills and notes — 67,565 — 67,565 Totals $ 7,244 $ 67,565 $ — $ 74,809 Fair value measurements at (in thousands) Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 15,076 $ — $ — $ 15,076 Totals $ 15,076 $ — $ — $ 15,076 U.S. government money market funds were valued by the Company based on quoted market prices, which represent a Level 1 measurement within the fair value hierarchy. U.S. treasury bills and notes were valued by the Company using quoted prices in active markets for similar securities, which represent a Level 2 measurement within the fair value hierarchy. During the years ended December 31, 2018 and 2019, there were no transfers between Level 1, Level 2 and Level 3.

Property and equipment, net

Property and equipment, net	12 Months Ended
	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Property and equipment, net	4. Property and equipment, net Property and equipment, net consisted of the following: December 31, (in thousands) 2018 2019 Laboratory equipment $ 3,053 $ 5,029 Computer equipment and software 261 740 Furniture and fixtures — 826 Leasehold improvements 1,561 12,993 Construction in progress — 4,487 4,875 24,075 Less: Accumulated depreciation and amortization (389 ) (2,230 ) $ 4,486 $ 21,845 Depreciation and amortization expense for the years ended December 31, 2018 and 2019 was $0.3 million and $1.9 million, respectively. At December 31, 2019, construction in progress was related to the build-out

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	4. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following: December 31, 2019 September 30, 2020 (in thousands) Accrued employee compensation and benefits $ 3,255 $ 3,798 Accrued external research and development expenses 1,344 2,037 Accrued professional fees 619 890 Deferred rent 1,370 1,380 Other 319 640 $ 6,907 $ 8,745	5. Accrued expenses Accrued expenses consisted of the following: December 31, (in thousands) 2018 2019 Accrued employee compensation and benefits $ 2,572 $ 3,255 Accrued external research and development expenses 1,284 1,344 Accrued professional fees 102 619 Deferred rent 4 1,370 Other 912 319 $ 4,874 $ 6,907

Convertible Preferred Stock

Convertible Preferred Stock	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Temporary Equity Disclosure [Abstract]
Convertible Preferred Stock	5. Convertible Preferred Stock The Company had issued Series A convertible preferred stock (the “Series A”), Series B convertible preferred stock (the “Series B”) and Series C convertible preferred stock (the “Series C”). Collectively the Series A, Series B and Series C are referred to as the Preferred Stock. On January 9, 2020, the Company issued and sold 19,936,296 shares of Series C at a price of $5.5914 per share for gross proceeds of $111.5 million. The Company incurred issuance costs in connection with this transaction of $2.6 million. Upon the closing of the IPO in June 2020, the Company’s Preferred Stock automatically converted into 27,094,085 shares of common stock.	6. Convertible preferred stock The Company has issued Series A convertible preferred stock (the “Series A”), Series B convertible preferred stock (the “Series B”) and Series C convertible preferred stock (the “Series C”). Collectively the Series A, Series B and Series C are referred to as the Preferred Stock. On February 22, 2018, the Company issued and sold 10,974,644 shares of Series B at a price of $9.1457 per share for gross proceeds of $100.4 million. The Company incurred issuance costs in connection with this transaction of $0.2 million. On January 9, 2020 (unaudited), the Company issued and sold 19,936,296 shares of Series C at a price of $5.5914 per share for gross proceeds of $111.5 million. The Company incurred issuance costs in connection with this transaction of $2.6 million. Upon issuance of each class of Preferred Stock, the Company assessed the embedded conversion and liquidation features of the shares and determined that such features did not require the Company to separately account for these features. The Company also concluded that no beneficial conversion feature existed on the issuance date of each class of Preferred Stock. Preferred Stock consisted of the following at December 31, 2018 and 2019: (in thousands except share amounts) Preferred Preferred Carrying Liquidation Common Series A Preferred Stock 15,451,020 15,451,020 $ 15,404 $ 15,451 8,747,675 Series B Preferred Stock 10,974,644 10,974,644 100,189 100,371 6,213,352 (1) 26,425,664 26,425,664 $ 115,593 $ 115,822 14,961,027 (1) (1) Upon issuance and sale of the Series C in January 2020 (see Note 13), the 10,974,644 outstanding shares of Series B became convertible into 7,059,406 shares of common stock and the 26,425,664 outstanding shares of Preferred Stock became convertible into 15,807,081 shares of common stock. The holders of Preferred Stock have the following rights and preferences: Voting The holders of Preferred Stock are entitled to vote, together with the holders of common stock, on matters submitted to stockholders for a vote. The holders of Preferred Stock are entitled to the number of votes equal to the number of shares of common stock into which each such share of Preferred Stock could convert. In addition, the holders of Series A, voting exclusively and as a separate class, are entitled to elect two directors of the Company. The holders of Series B, voting exclusively and as a separate class, are entitled to elect one director of the Company. Conversion Each share of Preferred Stock is convertible at the option of the holder at any time after the date of issuance. Each share of Preferred Stock will be automatically converted into shares of common stock at the applicable conversion ratio then in effect upon the closing of a firm commitment public offering of common stock with a price of at least $8.0496 per share and at least $40.0 million of gross proceeds to the Company. Shares of Series A will be automatically converted into shares of common stock at the applicable conversion ratio then in effect upon written consent of the holders of at least a majority of the then-outstanding shares of Series A. Shares of Series B will be automatically converted into shares of common stock at the applicable conversion ratio then in effect upon written consent of the holders of at least 57% of the then-outstanding shares of Series B. Shares of Series C will be automatically converted into shares of common stock at the applicable conversion ratio then in effect upon written consent of the holders of at least a majority of the then-outstanding shares of Series C. The conversion ratio of each series of Preferred Stock is determined by dividing the Original Issue Price of each series by the Conversion Price of each series. The Original Issue Price is $1.00 per share for Series A and $9.1457 per share for Series B. The Conversion Price at issuance was $1.7663 per share for Series A and $16.1540 per share (when rounded to the nearest ten-thousandth) 1.7663-for-one ten-thousandth), 1.7663-for-one Upon the issuance and sale of Series C in January 2020 at the Original Issue Price of $5.5914 per share, the Conversion Price of the Series B was adjusted to $14.2180 per share (when rounded to the nearest ten-thousandth) Dividends The holders of Preferred Stock are entitled to receive noncumulative dividends if and when declared by the Company’s board of directors at a rate of 6.0% of the Original Issuance Price per share of each series of Preferred Stock (subject to appropriate adjustment in the event of any stock split, stock dividend, combination or other similar recapitalization with respect to such shares) per annum. The Company may not declare, pay or set aside any dividends on shares of any other series of capital stock of the Company, other than dividends on common stock payable in common stock, unless the holders of the Preferred Stock first receive, or simultaneously receive, a dividend on each outstanding share of Preferred Stock in an amount equal to the greater of (i) 6.0% of the Original Issuance Price of each series of Preferred Stock (subject to appropriate adjustment in the event of any stock split, stock dividend, combination or other similar recapitalization with respect to such shares) per annum from the date of issuance of such shares, and (ii) (A) in the case of a dividend on common stock or any class or series of stock that is convertible into common stock, that dividend per share of Preferred Stock as would equal the product of (1) the dividend payable on each share of such class or series determined, if applicable, as if all shares of such class or series had been converted into common stock and (2) the number of shares of common stock issuable upon conversion of each share of Preferred Stock, or (B) in the case of a dividend on any class or series that is not convertible into common stock, at a rate per share of Preferred Stock determined by (1) dividing the amount of the dividend payable on each share of such class or series of capital stock by the Original Issue Price of such class or series of capital stock (subject to appropriate adjustment in the event of any stock dividend, stock split, combination of or other similar recapitalization affecting such shares) and (2) multiplying such fraction by an amount equal to the Original Issue Price of each series of Preferred Stock. If the Company declares, pays or sets aside, on the same date, a dividend on shares of more than one class or series of capital stock of the Company, the dividend payable to the holders of the Preferred Stock shall be calculated based upon the dividend on the class or series of capital stock that would result in the highest Preferred Stock dividend. No dividends were declared or paid during the years ended December 31, 2018 or 2019. Liquidation In the event of any voluntary or involuntary liquidation, dissolution or winding-up of Unless the holders of (i) at least a majority of the then-outstanding shares of Preferred Stock, voting together as a single class on an as-converted basis, as-converted as-converted

Common stock

Common stock	12 Months Ended
	Dec. 31, 2019
Common stock [Abstract]
Common stock	7. Common stock The voting, dividend and liquidation rights of the holders of the Company’s common stock are subject to and qualified by the rights, powers and preferences of the holders of the Preferred Stock set forth above. Each share of common stock entitles the holder to one vote, together with the holders of the Preferred Stock, on all matters submitted to the stockholders for a vote.

Equity

Equity	9 Months Ended
	Sep. 30, 2020
Equity [Abstract]
Equity	6. Equity As of September 30, 2020, the Company’s amended and restated certificate of incorporation authorizes the Company to issue 150,000,000 shares of common stock, par value $0.0001 per share, and 5,000,000 shares of preferred stock, par value $0.0001 per share, all of which preferred stock is undesignated. Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are not entitled to receive dividends, unless declared by the board of directors.

Stock-based compensation

Stock-based compensation	12 Months Ended
	Dec. 31, 2019
Share-based Payment Arrangement [Abstract]
Stock-based compensation	8. Stock-based compensation 2017 Stock Incentive Plan The Company’s 2017 Stock Incentive Plan (the “2017 Plan”) provides for the Company to grant incentive stock options or nonstatutory stock options, restricted stock, restricted stock units and other equity awards to employees, directors and consultants of the Company. The 2017 Plan is administered by the board of directors or, at the discretion of the board of directors, by a committee of the board of directors. The exercise prices, vesting and other restrictions on any award under the 2017 Plan are determined at the discretion of the board of directors, or its committee if so delegated. Stock options granted under the 2017 Plan with service-based vesting conditions generally vest over four years and expire after ten years. The exercise price for stock options granted is not less than the fair value of common stock as determined by the board of directors as of the date of grant. The total number of shares of common stock that may be issued under the 2017 Plan was 8,407,405 shares as of December 31, 2019, of which 773,652 shares remained available for future issuance. Shares that are expired, terminated, surrendered or canceled without having been fully exercised will be available for future awards under the 2017 Plan. Stock option valuation The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. For options with service-based vesting conditions, the expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future. The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of stock options granted: Stock options Year ended 2018 2019 Risk-free interest rate 2.70% 2.00% Expected volatility 76.0% 75.7% Expected dividend yield — — Expected term (in years) 6.1 6.0 The following table summarizes the Company’s stock option activity since December 31, 2018: Number of Weighted Weighted Aggregate Outstanding as of December 31, 2018 2,784,581 $ 4.05 9.4 $ 1,403 Granted 1,200,954 5.92 Exercised (34,740 ) 2.88 Forfeited (576,911 ) 3.89 Outstanding as of December 31, 2019 3,373,884 $ 4.76 8.79 $ 8,492 Vested and expected to vest as of December 31, 2019 3,373,884 $ 4.76 8.79 $ 8,492 Options exercisable as of December 31, 2019 852,647 $ 4.05 8.38 $ 2,735 The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had strike prices lower than the fair value of the Company’s common stock. The weighted average grant-date fair value of awards granted during the years ended December 31, 2018 and 2019 was $3.59 per share and $4.46 per share, respectively. There were no stock options exercised during the year ended December 31, 2018. The total intrinsic value of stock options exercised during the year ended December 31, 2019 was $0.1 million. Restricted common stock The Company issued 394,213 shares of service-based restricted common stock awards during the year ended December 31, 2018. The Company did not issue any restricted common stock awards during the year ended December 31, 2019. The following table summarizes the Company’s restricted common stock activity since December 31, 2018: Shares Weighted Unvested restricted common stock as of December 31, 2018 2,819,949 $ 1.24 Issued — — Vested (1,055,997 ) 1.30 Forfeited (58,764 ) 0.73 Unvested restricted common stock as of December 31, 2019 1,705,188 $ 1.22 The total fair value of restricted common stock vested during the years ended December 31, 2018 and 2019 was approximately $1.2 million and $7.0 million, respectively. Stock-Based Compensation The Company records compensation cost for all share-based payment arrangements, including employee, director and consultant stock options and restricted common stock. The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations and comprehensive loss: Year ended (in thousands) 2018 2019 Research and development expenses $ 2,093 $ 2,753 General and administrative expenses 1,395 1,454 $ 3,488 $ 4,207 As of December 31, 2019, total unrecognized compensation cost related to unvested stock-based awards was $10.9 million, which is expected to be recognized over a weighted average period of 2.4 years.

Income taxes

Income taxes	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income taxes	9. Income taxes For the years ended December 31, 2018 and 2019, the Company recorded no income tax benefits for the net operating losses incurred or for the research and development tax credits generated in each year, due to its uncertainty of realizing a benefit from those items. All of the Company’s operating losses since inception have been generated in the United States. A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year ended 2018 2019 Federal statutory income tax rate (21.0)% (21.0)% State income taxes, net of federal benefit (5.6) (5.9) Federal and state research and development tax credits (6.5) (5.1) Stock-based compensation expense 1.6 1.0 Other — 0.2 Change in deferred tax asset valuation allowance 31.5 30.8 Effective income tax rate 0.0% 0.0% Net deferred tax assets as of December 31, 2018 and 2019 consisted of the following: December 31, (in thousands) 2018 2019 Deferred tax assets: Net operating loss carryforwards $ 10,183 $ 25,023 Research and development tax credit carryforwards 2,878 5,998 Deferred rent — 4,740 Other 1,701 1,469 Total deferred tax assets 14,762 37,230 Deferred tax liabilities: Property and equipment (516 ) (4,117 ) Total deferred tax liabilities (516 ) (4,117 ) Valuation allowance (14,246 ) (33,113 ) Net deferred tax assets $ — $ — As of December 31, 2019, the Company had federal net operating loss carryforwards of $91.9 million, which may be available to offset future taxable income, of which $8.2 million of the total net operating loss carryforwards expire at various dates beginning in 2036, while the remaining $83.7 million do not expire but may be limited in their usage to an annual deduction equal to 80% of annual taxable income. As of December 31, 2019, the Company had state net operating loss carryforwards of $90.6 million, which may be available to offset future taxable income and expire at various dates beginning in 2036. In addition, as of December 31, 2019, the Company also had federal and state research and development tax credit carryforwards of $4.0 million and $2.5 million, respectively, which may be available to reduce future tax liabilities and expire at various dates beginning in 2036 and 2032, respectively. Utilization of the U.S. federal and state net operating loss carryforwards and research and development tax credit carryforwards may be subject to a substantial annual limitation under Section 382 and Section 383 of the Internal Revenue Code of 1986, and corresponding provisions of state law, due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income and tax liabilities. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain stockholders or public groups in the stock of a corporation by more than 50% over a three-year period. The Company has not conducted a study to assess whether a change of control has occurred or whether there have been multiple changes of control since inception due to the significant complexity and cost associated with such a study. If the Company has experienced a change of control, as defined by Section 382, at any time since inception, utilization of the net operating loss carryforwards or research and development tax credit carryforwards may be subject to an annual limitation, which is determined by first multiplying the value of the Company’s stock at the time of the ownership change by the applicable long-term tax-exempt rate, The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets, which consist primarily of net operating loss carryforwards and research and development tax credit carryforwards. Management has considered the Company’s history of cumulative net losses incurred since inception, estimated future taxable income and prudent and feasible tax planning strategies and has concluded that it is more likely than not that the Company will not realize the benefits of federal and state net deferred tax assets. Accordingly, a full valuation allowance has been established against the net deferred tax assets as of December 31, 2018 and 2019. The Company reevaluates the positive and negative evidence at each reporting period. The changes in the valuation allowance for deferred tax assets during the year ended December 31, 2018 and 2019 related primarily to the increases in net operating loss carryforwards and research and development tax credit carryforwards. The changes in the valuation allowance for 2018 and 2019 were as follows: Year ended (in thousands) 2018 2019 Valuation allowance as of beginning of year $ 2,976 $ 14,246 Increases recorded to income tax provision 11,270 18,867 Valuation allowance as of end of year $ 14,246 $ 33,113 The Company assesses the uncertainty in its income tax positions to determine whether a tax position of the Company is more likely than not to be sustained upon examination, including resolution of any related appeals of litigation processes, based on the technical merits of the position. For tax positions meeting the more-likely-than-not threshold, The Company files income tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations. The Company is open to future tax examination under statute from 2016 to the present.

License Agreements

License Agreements	12 Months Ended
	Dec. 31, 2019
License Agreements Disclosure [Abstract]
License Agreements	10. License agreements NIH The Company has an agreement with the U.S. Department of Health and Human Services, as represented by The National Heart, Lung, and Blood Institute, an Institute of the National Institutes of Health (“NIH”), entered into in 2017, pursuant to which NIH granted the Company a non-exclusive The Company is obligated to make future milestone payments of up to $0.4 million per licensed product upon the achievement of specified milestones as well as royalties on a licensed product-by-licensed country-by-country basis licensed product-by-licensed product country-by-country basis last-to-expire Unless terminated earlier, the agreement remains in effect until the last to expire of the licensed patent rights on a licensed product-by-licensed country-by-country basis. During each of the years ended December 31, 2018 and 2019, the Company recorded research and development expense of less than $0.1 million under this agreement. UMass The Company has an agreement with the University of Massachusetts as represented by and solely on behalf of its Medical School (“UMass”), entered into in 2017, pursuant to which UMass granted the Company an exclusive license, with the right to grant sublicenses, under the UMass intellectual property related to the Company’s ceDNA construct. The Company is obligated to make future milestone payments of up to $0.8 million per licensed product upon the achievement of specified milestones as well as royalties on a licensed product-by-licensed country-by-country basis low-to-mid non-refundable non-creditable licensed product-by-licensed product country-by-country basis last-to-expire As part of the arrangement, the Company agreed to issue 125,677 shares of its common stock to UMass. The Company recorded $0.1 million as research and development expense and additional-paid-in Unless terminated earlier, the agreement will continue until the last-to-expire During each of the years ended December 31, 2018 and 2019, the Company recorded research and development expense of less than $0.1 million under this agreement. Other license agreements The Company has other license agreements under which it may become subject to future additional fees and milestone payments.

Stock-Based Awards

Stock-Based Awards	9 Months Ended
	Sep. 30, 2020
Share-based Payment Arrangement [Abstract]
Stock-Based Awards	7. Stock-Based Awards Stock incentive plans The Company’s 2017 Stock Incentive Plan (the “2017 Plan”) provided for the Company to grant incentive stock options or nonstatutory stock options, restricted stock, restricted stock units and other equity awards to employees, directors and consultants of the Company. In January 2020, the number of shares of common stock authorized for issuance under the 2017 Plan was increased from 8,407,405 shares to 10,275,717 shares. In May 2020 the Company’s board of directors adopted, and in June 2020 the Company’s stockholders approved, the 2020 Stock Incentive Plan (the “2020 Plan” and, together with the 2017 Plan the “Plans”), which became effective on June 11, 2020. The 2020 Plan provides for the grant of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. Upon effectiveness of the 2020 Plan, the number of shares of common stock reserved for issuance under the 2020 Plan was the sum of (1) 2,547,698 shares; plus (2) the number of shares (up to a maximum of 7,173,014 shares) as was equal to the sum of (x) the number of shares of common stock reserved for issuance under the 2017 Plan that remained available for grant under the 2017 Plan on June 11, 2020 and (y) the number of shares of common stock subject to outstanding awards granted under the 2017 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right; plus (3) an annual increase, to be added on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2021 and continuing until, and including, the fiscal year ending December 31, 2030, equal to the lesser of (i) 4% of the number of shares of common stock outstanding on such date, and (ii) an amount determined by the board of directors. Upon the effectiveness of the 2020 Plan, the Company ceased granting additional awards under the 2017 Plan. The Plans are administered by the board of directors or, at the discretion of the board of directors, by a committee of the board of directors. The exercise prices, vesting and other restrictions on any award under the Plans are determined at the discretion of the board of directors, or its committee if so delegated. Stock options granted under the Plans with service-based vesting conditions generally vest over four years and expire after ten years. The exercise price for stock options granted is not less than the fair value of common stock as of the date of grant. Prior to the Company’s IPO, fair value of common stock was determined by the board of directors. Subsequent to the IPO, fair value of common stock will be based on quoted market prices. As of September 30, 2020, 2,640,244 shares remained available for future issuance under the 2020 Plan. Shares subject to outstanding awards granted under the Plans that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right will be available for future awards under the 2020 Plan. Grant of stock options During the nine months ended September 30, 2020, the Company granted service-based options to certain employees, directors and consultants for the purchase of 2,114,361 shares of common stock with a weighted average grant date fair value of $8.71 per share that vest over approximately four years. During the nine months ended September 30, 2020, the Company granted performance-based options to certain employees, directors and consultants for the purchase of 717,154 shares of common stock with a weighted average grant date fair value of $3.26 per share that vest upon the achievement of specified milestones. Employee stock purchase plan In May 2020 the Company’s board of directors adopted, and in June 2020 the Company’s stockholders approved, the 2020 Employee Stock Purchase Plan (the “2020 ESPP”), which became effective June 11, 2020. The 2020 ESPP will be administered by the Company’s board of directors or by a committee appointed by the board of directors. The 2020 ESPP initially provides participating employees with the opportunity to purchase up to an aggregate of 481,231 shares of common stock. The number of shares of common stock reserved for issuance under the 2020 ESPP will automatically increase on the first day of each fiscal year, beginning with the fiscal year commencing on January 1, 2021 and continuing for each fiscal year until, and including the fiscal year commencing on, January 1, 2030, in an amount equal to the lowest of (1) 1,302,157 shares of common stock, (2) 1% of the number of shares of common stock outstanding on such date, and (3) an amount determined by the board of directors. No offering periods have commenced under the 2020 ESPP and 481,231 shares remain available for issuance. Stock-based compensation Stock-based compensation expense was classified in the statements of operations and comprehensive loss as follows: Three Months Ended Nine Months Ended 2019 2020 2019 2020 (in thousands) Research and development expenses $ 688 $ 1,204 $ 2,039 $ 2,967 General and administrative expenses 392 957 1,083 2,066 $ 1,080 $ 2,161 $ 3,122 $ 5,033 As of September 30, 2020, total unrecognized compensation cost related to unvested stock-based awards was $23.6 million, which is expected to be recognized over a weighted average period of 3.2 years. Additionally, as of September 30, 2020, the Company has unrecognized compensation cost related to unvested stock-based awards with performance-based vesting conditions for which performance has not been deemed probable of $2.3 million.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	9. Commitments and Contingencies The Company leases its office and laboratory space under a noncancelable operating lease that was entered into in August 2018, as amended in June 2020, and expires in 2029. The Company has an option to extend for one additional term of five years Rent expense for the three months ended September 30, 2019 and 2020 was $2.5 million and $1.8 million, respectively. Rent expense for the nine months ended September 30, 2019 and 2020 was $6.0 million and $5.5 million, respectively. Future minimum lease payments as of September 30, 2020 are as follows: Year Ending December 31, (in thousands) 2020 (remaining 3 months) $ 1,745 2021 7,053 2022 7,251 2023 7,441 2024 7,679 Thereafter 35,451 $ 66,620 401(k) Plan The Company has a defined-contribution plan under Section 401(k) of the Internal Revenue Code of 1986 (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. Indemnification agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, contract research organizations, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and its executive officers that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. The Company has not incurred any material costs as a result of such indemnifications and is not currently aware of any indemnification claims. Legal proceedings The Company, from time to time, may be party to litigation arising in the ordinary course of business. The Company was not subject to any material legal proceedings during the three and nine months ended September 30, 2020.	11. Commitments and contingencies The Company leases its office and laboratory space under a noncancelable operating lease that was entered into in August 2018, as amended in July 2019, and expires in 2029. The Company has an option to extend for one additional term of five years at the greater of the then-current base rent, or the then-current fair market value. The lease agreement includes provisions for a free-rent period and annual rent increases, which are accrued or deferred as appropriate such that rent expense for the lease is recognized on a straight-line basis over the lease term (see Note 2). The Company is responsible for real estate taxes, maintenance, and other operating expenses applicable to the leased premises. The lease provides for a tenant improvement allowance, at the cost of the lessor, not to exceed $14.7 million. Costs incurred by the Company for tenant improvements but not yet reimbursed by the landlord are presented on the accompanying consolidated balance sheets as tenant receivable. As of December 31, 2018 and 2019, the Company had a tenant receivable of $1.3 million and $0.4 million, respectively. The Company posted a customary letter of credit in the amount of approximately $2.1 million as a security deposit. The letter of credit is subject to increase if the Company were to sublease any portion of the leased premises. Previously, the Company leased its office and laboratory space under noncancelable operating leases that expired during 2019. Rent expense for the years ended December 31, 2018 and 2019 was $3.5 million and $7.9 million, respectively. Future minimum lease payments as of December 31, 2019 are as follows: Year ending December 31, (in thousands) 2020 $ 6,867 2021 7,053 2022 7,251 2023 7,441 2024 7,679 Thereafter 35,451 $ 71,742 401(k) Plan The Company has a defined-contribution plan under Section 401(k) of the Internal Revenue Code of 1986 (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. Indemnification agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, contract research organizations, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and its executive officers that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. The Company has not incurred any material costs as a result of such indemnifications and is not currently aware of any indemnification claims. Legal proceedings The Company, from time to time, may be party to litigation arising in the ordinary course of business. The Company was not subject to any material legal proceedings during the years ended December 31, 2018 and 2019.

Net Loss per Share

Net Loss per Share	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Earnings Per Share [Abstract]
Net Loss per Share	8. Net Loss per Share The following potential dilutive securities, presented based on amounts outstanding at each period end, have been excluded from the calculation of diluted net loss per share because including them would have had an anti-dilutive impact: September 30, 2019 2020 Convertible preferred stock (as converted to common stock) 14,961,027 — Unvested restricted common stock 1,970,209 956,718 Stock options to purchase common stock 3,171,498 5,581,717 20,102,734 6,538,435	12. Net loss per share Net loss per share The Company has generated a net loss in all periods presented, therefore the basic and diluted net loss per share attributable to common stockholders are the same as the inclusion of the potentially dilutive securities would be anti-dilutive. The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated: Year ended December 31, 2018 2019 Convertible preferred stock (as converted to common stock) 14,961,027 14,961,027 Unvested restricted common stock 2,819,949 1,705,188 Stock options to purchase common stock 2,784,581 3,373,884 20,565,557 20,040,099

Subsequent events

Subsequent events	12 Months Ended
	Dec. 31, 2019
Subsequent Events [Abstract]
Subsequent events	13. Subsequent events The Company reviewed for subsequent events through April 9, 2020, the date the consolidated financial statements were issued and June 8, 2020, the date the revised consolidated financial statements were issued, to ensure that these consolidated financial statements include appropriate disclosure of events both recognized in the consolidated financial statements as of December 31, 2019 and events which occurred subsequently but were not recognized in the consolidated financial statements. The Company has concluded that no subsequent events have occurred that require disclosure, except as described below. (a) Issuance and sale of Series C In January 2020, the Company issued and sold 19,936,296 shares of Series C at a price of $5.5914 per share, for gross proceeds of $111.5 million. The terms of the Series C are substantially the same as the terms of the Series A and Series B except for the liquidation preference per share, which is equal to the per share price paid. In connection with the issuance, the Company increased the number of authorized shares of Preferred Stock from 26,425,664 shares to 46,361,960 shares. Because the price per share of the Series C in this transaction was lower than the Conversion Price of the Company’s Series B, in accordance with the Company’s certificate of incorporation, as amended and restated, the Conversion Price of Series B was adjusted from $16.1540 to $14.2180 per share (when rounded to the nearest ten-thousandth). (b) Increase in shares available for issuance under the 2017 Plan In January 2020, the number of shares of common stock authorized for issuance under the 2017 Plan was increased from 8,407,405 shares to 10,275,717 shares. (c) Grant of stock options under the 2017 Plan On March 5, 2020, the Company granted options with service-based vesting criteria for the purchase of an aggregate of 947,411 shares of common stock, at an exercise price of $5.16 per share. The aggregate grant-date fair value of these options was $3.4 million, which is expected to be recognized over approximately four years. On March 5, 2020, the Company granted options with performance-based vesting criteria for the purchase of an aggregate of 717,198 shares of common stock, at an exercise price of $5.16 per share. The aggregate grant-date fair value of these options was $2.3 million, which will be recognized once it becomes probable that the milestone will be achieved and the shares will vest. On March 31, 2020, the Company granted options with service-based vesting criteria for the purchase of an aggregate of 63,834 shares of common stock, at an exercise price of $5.25 per share. The aggregate grant-date fair value of these options was $0.2 million, which is expected to be recognized over approximately four years. (d) Reverse stock split The Company’s Board of Directors and stockholders approved a one-for-1.7663

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
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 contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these condensed consolidated financial statements include, but are not limited to, the accrual of research and development expenses and stock-based compensation expense. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results may differ from those estimates or assumptions.	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 contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses and stock-based compensation expense. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results may differ from those estimates or assumptions.
Unaudited interim financial information	Unaudited interim financial information The balance sheet at December 31, 2019 was derived from audited financial statements but does not include all disclosures required by GAAP. The accompanying unaudited financial statements as of September 30, 2020 and for the three and nine months ended September 30, 2019 and 2020 have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. These financial statements should be read in conjunction with the Company’s audited financial statements included elsewhere in this prospectus. In the opinion of management, all adjustments, consisting only of normal recurring adjustments necessary for a fair presentation of the Company’s financial position as of September 30, 2020, the results of operations for the three and nine months ended September 30, 2019 and 2020, and cash flows for the nine months ended September 30, 2019 and 2020 have been made. The results of operations for the three and nine months ended September 30, 2020 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2020 or any other period.
Concentrations of credit risk and of significant suppliers	Concentrations of credit risk and of significant suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents, and marketable securities. The Company believes it is not exposed to significant credit risk due to the financial strength of the depository institutions in which the cash and cash equivalents are held. The Company maintains its cash equivalents in money market funds that invest in U.S. treasury securities. The Company’s marketable securities as of September 30, 2020 consisted of U.S government treasury securities, U.S. government agency bonds, commercial paper and corporate debt securities. The Company has adopted an investment policy that limits the amounts the Company may invest in the securities of single issuer with the exclusion of the U.S. government. The Company has not experienced any credit losses and does not believe that it is subject to significant credit risk. The Company is dependent on a small number of third-party suppliers for its drug substance and drug product. In particular, the Company relies, and expects to continue to rely, on third-party suppliers for certain materials and components required for the production of any product candidates it may develop for its programs. These programs could be adversely affected by a significant interruption in the supply process.	Concentrations of credit risk and of significant suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. The Company has not experienced any other-than-temporary losses with respect to its cash and cash equivalents and does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on a small number of third-party suppliers for its drug substance and drug product. In particular, the Company relies, and expects to continue to rely, on third-party suppliers for certain materials and components required for the production of any product candidates it may develop for its programs. These programs could be adversely affected by a significant interruption in the supply process.
Deferred offering costs		Deferred offering costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity paid-in
Cash equivalents		Cash equivalents The Company considers all highly liquid investments with a remaining maturity when purchased of three months or less to be cash equivalents.
Restricted cash		Restricted cash Amounts included in restricted cash represent amounts pledged as collateral for letters of credit required for security deposits on the Company’s leased facilities. These amounts are classified as restricted cash (current and noncurrent) on the Company’s consolidated balance sheets.
Marketable securities		Marketable securities The Company’s marketable securities, which consisted of debt securities as of December 31, 2018, are classified as available-for-sale and on available-for-sale debt The Company evaluates its marketable securities with unrealized losses for other-than-temporary impairment. When assessing marketable securities for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, how long the market value of the investment has been less than its original cost, the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary,” the Company reduces the investment to fair value through a charge to the statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented.
Fair value measurements	Fair value measurements Certain assets and liabilities of the Company are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) 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. Financial assets and financial liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents and marketable securities are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities.	Fair value measurements Certain assets and liabilities of the Company are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) 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. Financial assets and financial liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents and marketable securities are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s tenant receivable, accounts payable and accrued expenses approximate their fair values due to the short-term nature of these assets and liabilities.
Deferred rent		Deferred rent Payment escalations, rent holidays and other lease incentives that may be included in lease agreements are accrued or deferred as appropriate such that rent expense for each lease is recognized on a straight-line basis over the respective lease term. Adjustments for such items are recorded as deferred rent and amortized over the respective lease terms.
Property and equipment		Property and equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset as follows: Estimated useful life Laboratory equipment 5 years Computer equipment and software 3 years Furniture and fixtures 5 years Leasehold improvements Shorter of remaining life of lease or useful life Costs for capital assets not yet placed into service are capitalized as construction-in-progress
Impairment of long-lived assets		Impairment of long-lived assets Long-lived assets consist of property and equipment. The Company evaluates the recoverability of its long-lived assets when circumstances indicate that an event of impairment may have occurred. The Company recognizes an impairment loss only if the carrying amount of a long-lived asset is not recoverable based on its undiscounted future cash flows. Impairment is measured based on the difference between the carrying value of the related assets and the fair value of such assets. The Company did not record any impairment losses on long-lived assets during the years ended December 31, 2018 or 2019.
Classification and accretion of convertible preferred stock		Classification and accretion of convertible preferred stock The Company’s convertible preferred stock is classified outside of stockholders’ equity (deficit) on the consolidated balance sheets because the holders of such shares have redemption rights in the event of a deemed liquidation that, in certain situations, is not solely within the control of the Company and would require the redemption of the then-outstanding convertible preferred stock. A deemed liquidation includes Liquidation Events as that term is defined in Note 6. The Company’s Series A and Series B convertible preferred stock are not redeemable, except in the event of a deemed liquidation (see Note 6). Because the occurrence of a deemed liquidation event is not currently probable, the carrying values of the convertible preferred stock are not being accreted to their redemption values. If a deemed liquidation event became probable, the carrying values of the convertible preferred stock would be accreted to redemption values.
Segment information		Segment information The Company has determined that its chief executive officer is the chief operating decision maker (“CODM”). The CODM reviews financial information presented on a consolidated basis. Resource allocation decisions are made by the CODM based on consolidated results. There are no segment managers who are held accountable by the CODM for operations, operating results, and planning for levels or components below the consolidated unit level. As such, the Company has concluded that it operates as one segment. All long-lived assets are located in the United States.
Revenue recognition		Revenue recognition To date, the Company has derived its revenue by providing services to pharmaceutical and life sciences companies. On January 1, 2019, the Company adopted ASU 2014-09, The Company only applies the five-step model to contracts when collectability of the consideration to which the Company is entitled in exchange for the goods or services it transfers to the customer is determined to be probable. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is that the period between payment by the customer and the transfer of the promised goods or services to the customer will be one year or less. The Company recognizes revenue when it satisfies its performance obligations by delivering the services to its customers in an amount that reflects the consideration to which it expects to be entitled in exchange for those services. Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue on the accompanying balance sheets. Amounts expected to be recognized as revenue within 12 months of the balance sheet date are classified as current deferred revenue. The Company had no deferred revenue as of December 31, 2019. The Company expenses incremental costs of obtaining a contract as and when incurred if the expected amortization period of the asset that would have been recognized is one year or less or the amount is immaterial. As of December 31, 2019, the Company had not capitalized any costs to obtain contracts.
Research and development costs		Research and development costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries and bonuses, stock-based compensation, employee benefits, facilities costs, laboratory supplies, depreciation and amortization, manufacturing expenses and external costs of vendors engaged to conduct preclinical development activities as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology are expensed as research and development expenses in the period in which they are incurred. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed.
Research and manufacturing contract costs and accruals		Research and manufacturing contract costs and accruals The Company has entered into various research and development and manufacturing contracts. These agreements are generally cancelable, and related payments are recorded as the corresponding expenses are incurred. The Company records accruals for estimated ongoing costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the research studies and manufacturing activities, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.
Patent costs		Patent costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.
Stock-based compensation		Stock-based compensation The Company measures stock options with service-based vesting or performance-based vesting granted to employees, non-employees Compensation expense for awards with service-based vesting is generally recognized over the vesting period of the award using the straight-line method to record the expense. The Company uses the graded-vesting method to record the expense of awards with both service-based and performance-based vesting conditions, commencing once achievement of the performance condition becomes probable. The Company accounts for forfeitures of share-based awards as they occur. The Company classifies stock-based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified. Prior to the adoption of ASU 2018-07 non-employees non-employees.
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 years ended December 31, 2018 and 2019, the Company’s only element of other comprehensive loss was unrealized gains (losses) on marketable securities.
Income taxes		Income taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process deemed more-likely-than-not to
Net loss per share		Net loss per share The Company follows the two-class two-class two-class Basic net income (loss) per share attributable to common stockholders is computed by dividing net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding for the period. Diluted net income (loss) per share attributable to common stockholders is computed by adjusting net income (loss) attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common stockholders is computed by dividing the diluted net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares. The Company’s participating securities contractually entitle the holders of such shares to participate in dividends but do not contractually require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss, such losses are not allocated to such participating securities. In periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. The Company reported a net loss attributable to common stockholders for the years ended December 31, 2018 and 2019.
Recently issued accounting pronouncements	Recently adopted accounting pronouncements In August 2018, the FASB issued ASU 2018-13, 2018-13 2018-03 Recently issued accounting pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies that the Company adopts as of the specified effective date. The Company qualifies as an “emerging growth company” as defined in the Jumpstart Our Business Startups Act of 2012 and has elected not to “opt out” of the extended transition related to complying with new or revised accounting standards, which means that when a standard is issued or revised and it has different application dates for public and nonpublic companies, the Company can adopt the new or revised standard at the time nonpublic companies adopt the new or revised standard and can do so until such time that the Company either (i) irrevocably elects to “opt out” of such extended transition period or (ii) no longer qualifies as an emerging growth company. In February 2016, the FASB issued ASU 2016-02, 2018-11, 2019-10, 2020-05, one-year 2016-02 right-of-use In June 2016, the FASB issued ASU 2016-13, available-for-sale. 2019-10,	Recently adopted accounting pronouncements In May 2014, the FASB issued ASU 2014-09, 2014-09 2014-09 In June 2018, the FASB issued ASU 2018-07, ASU 2018-07 was For non-public entities, ASU 2018-07 is ASU 2014-09. The adopted ASU 2018-07 on In August 2018, the FASB issued ASU 2018-13, 2018-13 2018-03 Recently issued accounting pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies that the Company adopts as of the specified effective date. The Company qualifies as an “emerging growth company” as defined in the Jumpstart Our Business Startups Act of 2012 and has elected not to “opt out” of the extended transition related to complying with new or revised accounting standards, which means that when a standard is issued or revised and it has different application dates for public and nonpublic companies, the Company can adopt the new or revised standard at the time nonpublic companies adopt the new or revised standard and can do so until such time that the Company either (i) irrevocably elects to “opt out” of such extended transition period or (ii) no longer qualifies as an emerging growth company. In February 2016, the FASB issued ASU 2016-02, 2018-11, 2019-10, entities to annual reporting periods beginning after December 15, 2020, and interim periods within fiscal years beginning after December 15, 2021. Early adoption continues to be allowed. The Company will adopt ASU 2016-02 right-of-use In June 2016, the FASB issued ASU 2016-13, available-for-sale. 2019-10,

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of Property and equipment	Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset as follows: Estimated useful life Laboratory equipment 5 years Computer equipment and software 3 years Furniture and fixtures 5 years Leasehold improvements Shorter of remaining life of lease or useful life

Fair Value of Financial Assets

Fair Value of Financial Assets (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Summary of marketable securities	The Company did not have marketable securities at December 31, 2019. As of September 30, 2020, marketable securities by security type consisted of the following: Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value (in thousands) U.S. treasury securities $ 132,184 $ 16 $ — $ 132,200 U.S. government agency bonds 11,995 2 — 11,997 Commercial paper 59,891 — — 59,891 Corporate debt securities 17,665 8 — 17,673 Totals $ 221,735 $ 26 $ — $ 221,761	Marketable securities by security type consisted of the following: December 31, 2018 (in thousands) Amortized Gross Gross Fair U.S. treasury bills and notes $ 67,574 $ — $ (9 ) $ 67,565 $ 67,574 $ — $ (9 ) $ 67,565
Summary of assets measured at fair value on a recurring basis	The following tables present the Company’s assets that are measured at fair value on a recurring basis and indicate the level within the fair value hierarchy of the valuation techniques the Company utilized to determine such fair value, which is described further within Note 2, Summary of Significant Accounting Policies: Fair Value Measurements at Level 1 Level 2 Level 3 Total (in thousands) Cash equivalents: Money market funds $ 15,076 $ — $ — $ 15,076 Totals $ 15,076 $ — $ — $ 15,076 Fair Value Measurements at September 30, Level 1 Level 2 Level 3 Total (in thousands) Cash equivalents: Money market funds $ 57,394 $ — $ — $ 57,394 Marketable securities: U.S. treasury securities — 132,200 — 132,200 U.S. government agency bonds — 11,997 — 11,997 Commercial paper — 59,891 — 59,891 Corporate debt securities — 17,673 — 17,673 Totals $ 57,394 $ 221,761 $ — $ 279,155	The following tables present the Company’s assets that are measured at fair value on a recurring basis and indicate the level within the fair value hierarchy of the valuation techniques the Company utilized to determine such fair value, which is described further within Note 2, Fair value measurements at December 31, (in thousands) Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 7,244 $ — $ — $ 7,244 Marketable securities U.S. treasury bills and notes — 67,565 — 67,565 Totals $ 7,244 $ 67,565 $ — $ 74,809 Fair value measurements at (in thousands) Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 15,076 $ — $ — $ 15,076 Totals $ 15,076 $ — $ — $ 15,076

Property and equipment, net (Ta

Property and equipment, net (Tables)	12 Months Ended
	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Summary of Property and equipment, net	Property and equipment, net consisted of the following: December 31, (in thousands) 2018 2019 Laboratory equipment $ 3,053 $ 5,029 Computer equipment and software 261 740 Furniture and fixtures — 826 Leasehold improvements 1,561 12,993 Construction in progress — 4,487 4,875 24,075 Less: Accumulated depreciation and amortization (389 ) (2,230 ) $ 4,486 $ 21,845

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Summary of Accrued expenses and other current liabilities	Accrued expenses and other current liabilities consisted of the following: December 31, 2019 September 30, 2020 (in thousands) Accrued employee compensation and benefits $ 3,255 $ 3,798 Accrued external research and development expenses 1,344 2,037 Accrued professional fees 619 890 Deferred rent 1,370 1,380 Other 319 640 $ 6,907 $ 8,745	Accrued expenses consisted of the following: December 31, (in thousands) 2018 2019 Accrued employee compensation and benefits $ 2,572 $ 3,255 Accrued external research and development expenses 1,284 1,344 Accrued professional fees 102 619 Deferred rent 4 1,370 Other 912 319 $ 4,874 $ 6,907

Convertible Preferred Stock (Ta

Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2019
Temporary Equity Disclosure [Abstract]
Summary of Preferred Stock	Preferred Stock consisted of the following at December 31, 2018 and 2019: (in thousands except share amounts) Preferred Preferred Carrying Liquidation Common Series A Preferred Stock 15,451,020 15,451,020 $ 15,404 $ 15,451 8,747,675 Series B Preferred Stock 10,974,644 10,974,644 100,189 100,371 6,213,352 (1) 26,425,664 26,425,664 $ 115,593 $ 115,822 14,961,027 (1) (1) Upon issuance and sale of the Series C in January 2020 (see Note 13), the 10,974,644 outstanding shares of Series B became convertible into 7,059,406 shares of common stock and the 26,425,664 outstanding shares of Preferred Stock became convertible into 15,807,081 shares of common stock.

Stock-based compensation (Table

Stock-based compensation (Tables)	12 Months Ended
	Dec. 31, 2019
Share-based Payment Arrangement [Abstract]
Summary of Inputs Used in Black-Scholes Option Pricing Model	The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of stock options granted: Stock options Year ended 2018 2019 Risk-free interest rate 2.70% 2.00% Expected volatility 76.0% 75.7% Expected dividend yield — — Expected term (in years) 6.1 6.0
Summarizes the Company's stock option activity	The following table summarizes the Company’s stock option activity since December 31, 2018: Number of Weighted Weighted Aggregate Outstanding as of December 31, 2018 2,784,581 $ 4.05 9.4 $ 1,403 Granted 1,200,954 5.92 Exercised (34,740 ) 2.88 Forfeited (576,911 ) 3.89 Outstanding as of December 31, 2019 3,373,884 $ 4.76 8.79 $ 8,492 Vested and expected to vest as of December 31, 2019 3,373,884 $ 4.76 8.79 $ 8,492 Options exercisable as of December 31, 2019 852,647 $ 4.05 8.38 $ 2,735
summarizes the Company's restricted common stock activity	The following table summarizes the Company’s restricted common stock activity since December 31, 2018: Shares Weighted Unvested restricted common stock as of December 31, 2018 2,819,949 $ 1.24 Issued — — Vested (1,055,997 ) 1.30 Forfeited (58,764 ) 0.73 Unvested restricted common stock as of December 31, 2019 1,705,188 $ 1.22
Summary of stock-based compensation expense was classified in the statements of operations and comprehensive loss	The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations and comprehensive loss: Year ended (in thousands) 2018 2019 Research and development expenses $ 2,093 $ 2,753 General and administrative expenses 1,395 1,454 $ 3,488 $ 4,207

Income taxes (Tables)

Income taxes (Tables)	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Summary of reconciliation of the U.S. federal statutory income tax rate	A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year ended 2018 2019 Federal statutory income tax rate (21.0)% (21.0)% State income taxes, net of federal benefit (5.6) (5.9) Federal and state research and development tax credits (6.5) (5.1) Stock-based compensation expense 1.6 1.0 Other — 0.2 Change in deferred tax asset valuation allowance 31.5 30.8 Effective income tax rate 0.0% 0.0%
Summary of Net deferred tax assets	Net deferred tax assets as of December 31, 2018 and 2019 consisted of the following: December 31, (in thousands) 2018 2019 Deferred tax assets: Net operating loss carryforwards $ 10,183 $ 25,023 Research and development tax credit carryforwards 2,878 5,998 Deferred rent — 4,740 Other 1,701 1,469 Total deferred tax assets 14,762 37,230 Deferred tax liabilities: Property and equipment (516 ) (4,117 ) Total deferred tax liabilities (516 ) (4,117 ) Valuation allowance (14,246 ) (33,113 ) Net deferred tax assets $ — $ —
Summary of changes in the valuation allowance for deferred tax assets	The changes in the valuation allowance for deferred tax assets during the year ended December 31, 2018 and 2019 related primarily to the increases in net operating loss carryforwards and research and development tax credit carryforwards. The changes in the valuation allowance for 2018 and 2019 were as follows: Year ended (in thousands) 2018 2019 Valuation allowance as of beginning of year $ 2,976 $ 14,246 Increases recorded to income tax provision 11,270 18,867 Valuation allowance as of end of year $ 14,246 $ 33,113

Stock-Based Awards (Tables)

Stock-Based Awards (Tables)	9 Months Ended
	Sep. 30, 2020
Share-based Payment Arrangement [Abstract]
Summary of stock-based compensation expense was classified in the statements of operations and comprehensive loss	Stock-based compensation expense was classified in the statements of operations and comprehensive loss as follows: Three Months Ended Nine Months Ended 2019 2020 2019 2020 (in thousands) Research and development expenses $ 688 $ 1,204 $ 2,039 $ 2,967 General and administrative expenses 392 957 1,083 2,066 $ 1,080 $ 2,161 $ 3,122 $ 5,033

Net Loss per Share (Tables)

Net Loss per Share (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Earnings Per Share [Abstract]
Summary of potential dilutive securities, presented based on amounts outstanding	The following potential dilutive securities, presented based on amounts outstanding at each period end, have been excluded from the calculation of diluted net loss per share because including them would have had an anti-dilutive impact: September 30, 2019 2020 Convertible preferred stock (as converted to common stock) 14,961,027 — Unvested restricted common stock 1,970,209 956,718 Stock options to purchase common stock 3,171,498 5,581,717 20,102,734 6,538,435	The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated: Year ended December 31, 2018 2019 Convertible preferred stock (as converted to common stock) 14,961,027 14,961,027 Unvested restricted common stock 2,819,949 1,705,188 Stock options to purchase common stock 2,784,581 3,373,884 20,565,557 20,040,099

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Summary of future minimum lease payments	Future minimum lease payments as of September 30, 2020 are as follows: Year Ending December 31, (in thousands) 2020 (remaining 3 months) $ 1,745 2021 7,053 2022 7,251 2023 7,441 2024 7,679 Thereafter 35,451 $ 66,620	Future minimum lease payments as of December 31, 2019 are as follows: Year ending December 31, (in thousands) 2020 $ 6,867 2021 7,053 2022 7,251 2023 7,441 2024 7,679 Thereafter 35,451 $ 71,742

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jun. 16, 2020	Jan. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Jan. 09, 2020
Date of incorporation					Oct. 21, 2016
Net loss			$ (20,849)	$ (16,128)	$ (56,297)	$ (45,254)	$ (61,317)	$ (35,803)
Used in BS			$ (164,749)		(164,749)		$ (108,452)	$ (47,135)
Proceeds from issuance of convertible preferred stock					$ 108,854
Series C Convertible Preferred Stock [Member]
Shares issued price		$ 5.5914							$ 5.5914
Proceeds from issuance of convertible preferred stock		$ 111,500
Common Stock [Member]
Shares issued during the period					12,105,263
Common Stock [Member] | IPO [Member]
Shares issued during the period	12,105,263
Shares issued price	$ 19
Sale of Stock, Consideration Received on Transaction	$ 210,700
Common Stock [Member] | Over-Allotment Option [Member]
Shares issued during the period	1,578,947

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)
Accounting Policies [Line Items]
Income Tax rate settlement	50.00%
Series C Convertible Preferred Stock [Member]
Accounting Policies [Line Items]
Deferred offering costs	$ 0.1

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Summary Of Property And Equipment (Detail)	12 Months Ended
	Dec. 31, 2019
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	5 years
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	3 years
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	5 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Useful life	Shorter of remaining life of lease or useful life

Fair Value of Financial Asset_2

Fair Value of Financial Assets - Summary of marketable securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2018
Marketable Securities [Line Items]
Gross unrealized losses	$ 0	$ (9)
Fair Value, Recurring [Member]
Marketable Securities [Line Items]
Amortized cost	221,735	67,574
Gross unrealized gain	26	0
Estimated fair value	221,761	67,565
Fair Value, Recurring [Member] | U.S. treasury securities
Marketable Securities [Line Items]
Amortized cost	132,184	67,574
Gross unrealized gain	16	0
Gross unrealized losses	0	(9)
Estimated fair value	132,200	$ 67,565
Fair Value, Recurring [Member] | U.S. government agency bonds
Marketable Securities [Line Items]
Amortized cost	11,995
Gross unrealized gain	2
Estimated fair value	11,997
Fair Value, Recurring [Member] | Commercial paper
Marketable Securities [Line Items]
Amortized cost	59,891
Estimated fair value	59,891
Fair Value, Recurring [Member] | Corporate debt securities
Marketable Securities [Line Items]
Amortized cost	17,665
Gross unrealized gain	8
Estimated fair value	$ 17,673

Fair Value of Financial Asset_3

Fair Value of Financial Assets - Summary of assets measured at fair value on a recurring basis (Detail) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash equivalents:
Totals	$ 279,155	$ 15,076	$ 74,809
Marketable securities:
Marketable securities	221,761		67,565
Level 1
Cash equivalents:
Totals	57,394	15,076	7,244
Level 2
Cash equivalents:
Totals	221,761		67,565
Money Market Funds
Cash equivalents:
Cash equivalents	57,394	15,076	7,244
Money Market Funds | Level 1
Cash equivalents:
Cash equivalents	57,394	$ 15,076	7,244
U.S. treasury securities
Marketable securities:
Marketable securities	132,200		67,565
U.S. treasury securities | Level 2
Marketable securities:
Marketable securities	132,200		$ 67,565
U.S. government agency bonds
Marketable securities:
Marketable securities	11,997
U.S. government agency bonds | Level 2
Marketable securities:
Marketable securities	11,997
Commercial paper
Marketable securities:
Marketable securities	59,891
Commercial paper | Level 2
Marketable securities:
Marketable securities	59,891
Corporate debt securities
Marketable securities:
Marketable securities	17,673
Corporate debt securities | Level 2
Marketable securities:
Marketable securities	$ 17,673

Property and equipment, net - S

Property and equipment, net - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross		$ 24,075	$ 4,875
Less: Accumulated depreciation and amortization		(2,230)	(389)
Property, Plant and Equipment, Net	$ 22,888	21,845	4,486
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross		5,029	3,053
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross		740	261
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross		826
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross		12,993	$ 1,561
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross		$ 4,487

Property and equipment, net - A

Property and equipment, net - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment, Net, by Type [Abstract]
Depreciation and amortization expense	$ 2,441	$ 1,185	$ 1,864	$ 347

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities - Summary of Accrued expenses and other current liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Accrued Expenses And Other Liabilities Current [Line Items]
Accrued employee compensation and benefits	$ 3,798	$ 3,255	$ 2,572
Accrued external research and development expenses	2,037	1,344	1,284
Accrued professional fees	890	619	102
Deferred rent	1,380	1,370	4
Other	640	319	912
Total	$ 8,745	$ 6,907	$ 4,874

Income taxes - Additional Infor

Income taxes - Additional Information (Detail) $ in Millions	Dec. 31, 2019USD ($)	Dec. 31, 2019USD ($)
Income Tax [Line Items]
Net operating loss carry forwards	$ 8.2	$ 8.2
Operating Loss Carry forward Remaining Amount	$ 83.7	83.7
Percentage on annual taxable income	80.00%
Tax Credit Carryforward, Amount	$ 2.5	$ 2.5
Percentage of change in ownership	50.00%
Unrecognized Tax Benefits of income tax rate	50.00%
Tax Credit Carry forward, Expiration Year		2036
Operating Loss Carry forwards, Expiration Year		2036
Federal income tax Authority [Member]
Income Tax [Line Items]
Net operating loss carry forwards	$ 91.9	$ 91.9
State and Local Jurisdiction [Member]
Income Tax [Line Items]
Net operating loss carry forwards	90.6	90.6
Federal and state research and development tax credit carry forwards [Member]
Income Tax [Line Items]
Tax Credit Carryforward, Amount	$ 4	$ 4

Income taxes - Summary of Recon

Income taxes - Summary of Reconciliation of the U.S. Federal Statutory Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax rate	(21.00%)	(21.00%)
State income taxes, net of federal benefit	(5.90%)	(5.60%)
Federal and state research and development tax credits	(5.10%)	(6.50%)
Stock-based compensation expense	1.00%	1.60%
Other	0.20%	0.00%
Change in deferred tax asset valuation allowance	30.80%	31.50%
Effective income tax rate	0.00%	0.00%

Income taxes - Summary of Net D

Income taxes - Summary of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforwards	$ 25,023	$ 10,183
Research and development tax credit carryforwards	5,998	2,878
Deferred rent	4,740	0
Other	1,469	1,701
Total deferred tax assets	37,230	14,762
Deferred tax liabilities:
Property and equipment	(4,117)	(516)
Total deferred tax liabilities	(4,117)	(516)
Valuation allowance	(33,113)	(14,246)
Net deferred tax assets	$ 0	$ 0

Income taxes - Summary of Chang

Income taxes - Summary of Changes in the Valuation Allowance for Deferred Tax Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Valuation Allowance [Abstract]
Valuation allowance as of beginning of year	$ 14,246	$ 2,976
Increases recorded to income tax provision	18,867	11,270
Valuation allowance as of end of year	$ 33,113	$ 14,246

License Agreements - Additional

License Agreements - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
License Agreements Disclosure [Line Items]
Research and Development Expense		$ 15,308	$ 13,447	$ 42,158	$ 37,403	$ 50,134	$ 28,152
NIH [Member]
License Agreements Disclosure [Line Items]
Research and Development Expense						100	100
NIH [Member] | License Agreement Terms [Member]
License Agreements Disclosure [Line Items]
Future milestone obligation	$ 400					400
Percentage of increase decrease in net sales	25.00%
UMass [Member] | License Agreement Terms [Member]
License Agreements Disclosure [Line Items]
Future milestone obligation	$ 800					$ 800
Shares Issued For Services						125,677
Research and Development Expense						$ 100	$ 100
Adjustments to Additional Paid in Capital, Other						$ 100

Equity - Additional Information

Equity - Additional Information (Detail) - $ / shares	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Equity [Abstract]
Common stock, shares authorized	150,000,000	46,750,000	46,750,000
Common stock, par value per share	$ 0.0001	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	0
Preferred stock, par value per share	$ 0.0001	$ 0.0001

Stock-Based Awards - Additional

Stock-Based Awards - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Jun. 11, 2020	Sep. 30, 2020	Dec. 31, 2019	Jan. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compenation by share based payment arrangement number of options granted			1,200,954
Share based compenation by share based payment arrangement number of options granted weighted average exercise price per share			$ 5.92
Period of vesting of employee stock options		4 years
Unrecognised stock compensation on account of unvested options		$ 23.6	$ 10.9
Unrecognised stock compensation on account of unvested options remaining period over which the compensation is to be recognised		3 years 2 months 12 days	2 years 4 months 24 days
Unvested Stock Awards for Which Performance is not Probable
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognised stock compensation on account of unvested options		$ 2.3
Service Based Options [Member] | Certain Employees Directors and Consultants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation vesting period service based		4 years
Share based compenation by share based payment arrangement number of options granted		2,114,361
Share based compenation by share based payment arrangement number of options granted weighted average exercise price per share		$ 8.71
Performance Based Options [Member] | Certain Employees Directors and Consultants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compenation by share based payment arrangement number of options granted		717,154
Share based compenation by share based payment arrangement number of options granted weighted average exercise price per share		$ 3.26
2020 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock reserved for future issuance	2,547,698
Stock based compensation period of expiry		10 years
Common stock shares available for future issuance		2,640,244
2020 Stock Incentive Plan | Maximum | Additional From 2017 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock reserved for future issuance	7,173,014
2020 Stock Incentive Plan | Maximum | Annual Increase 2021 To 2030
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of common stock shares additionally reserved for issuance	4.00%
Employee Stock Purchase Plan 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation by share based payment arrangement number of shares authorized for issuance	481,231
Common stock shares available for future issuance		481,231
Employee Stock Purchase Plan 2020 | Maximum | Annual Increase 2021 To 2030
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of common stock shares additionally reserved for issuance	1.00%
Number of share instruments newly issued under a share-based compensation plan	1,302,157
Common Stock [Member] | 2017 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation by share based payment arrangement number of shares authorized for issuance		10,275,717	8,407,405	10,275,717

Stock-Based Awards - Summary of

Stock-Based Awards - Summary of Stock-Based Compensation Expense was Classified in the Statements of Operations and Comprehensive Loss (Detail) - 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	$ 2,161	$ 1,080	$ 5,033	$ 3,122
Research and development expense [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	1,204	688	2,967	2,039
General and administrative expenses
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 957	$ 392	$ 2,066	$ 1,083

Stock-based compensation - Summ

Stock-based compensation - Summary of Inputs Used in Black-Scholes Option Pricing Model (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.00%	2.70%
Expected volatility	75.70%	76.00%
Expected term (in years)	6 years	6 years 1 month 6 days

Stock-based compensation - Su_2

Stock-based compensation - Summarizes the Company's Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares Outstanding , Beginning	2,784,581
Number of shares | Granted	1,200,954
Number of shares | Exercised	(34,740)
Number of shares | Forfeited	(576,911)
Number of shares Outstanding , Ending	3,373,884	2,784,581
Number of shares | Vested and expected to vest	3,373,884
Number of shares | Options exercisable	852,647
Weighted average exercise price Outstanding , Beginning	$ 4.05
Weighted average exercise price | Granted	5.92
Weighted average exercise price | Exercised	2.88
Weighted average exercise price | Forfeited	3.89
Weighted average exercise price Outstanding , Ending	4.76	$ 4.05
Weighted average exercise price | Vested and expected to vest	4.76
Weighted average exercise price | Options exercisable	$ 4.05
Weighted average contractual term (in years) Outstanding , Beginning	8 years 9 months 14 days	9 years 4 months 24 days
Weighted average contractual term (in years) Outstanding , Ending	8 years 9 months 14 days	9 years 4 months 24 days
Weighted average contractual term (in years) | Vested and expected to vest	8 years 9 months 14 days
Weighted average contractual term (in years) | Options exercisable	8 years 4 months 17 days
Aggregate intrinsic value (in thousands) Outstanding	$ 8,492	$ 1,403
Aggregate intrinsic value (in thousands) | Vested and expected to vest	8,492
Aggregate intrinsic value (in thousands) | Options exercisable	$ 2,735

Stock-based compensation - Su_3

Stock-based compensation - Summarizes the Company's Restricted Common Stock Activity (Detail)	12 Months Ended
	Dec. 31, 2019$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted common stock, Beginning | Shares | shares	2,819,949
Vested | Shares | shares	(1,055,997)
Forfeited | Shares | shares	(58,764)
Unvested restricted common stock, Ending | Shares | shares	1,705,188
Weighted average grant date fair value of Unvested restricted common stock, Beginning | $ / shares	$ 1.24
Weighted average grant date fair value of Unvested restricted common stock, Vested | $ / shares	1.30
Weighted average grant date fair value of Unvested restricted common stock, Forfeited | $ / shares	0.73
Weighted average grant date fair value of Unvested restricted common stock, Ending | $ / shares	$ 1.22

Stock-based compensation - Su_4

Stock-based compensation - Summary of Stock-Based Compensation Expense was Classified in the Statements of Operations and Comprehensive Loss (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 4,207	$ 3,488
Research and development expense [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	2,753	2,093
General and administrative expenses
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 1,454	$ 1,395

Stock-based compensation - Addi

Stock-based compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognised stock compensation on account of unvested options	$ 23.6	$ 10.9
Unrecognised stock compensation on account of unvested options remaining period over which the compensation is to be recognised	3 years 2 months 12 days	2 years 4 months 24 days
weighted average grant-date fair value		$ 1.30
Share based compenation by share based payment arrangement number of options granted		1,200,954
Common Stock [Member] | 2017 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation by share based payment arrangement number of shares authorised for issuance		8,407,405
Stock reserved for future issuance		773,652
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
fair value of restricted common stock		$ 7	$ 1.2
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
weighted average grant-date fair value		$ 4.46	$ 3.59
No of stock option exercised		$ 0.1	$ 0
No of Instrinic stock option exercised		$ 0.1	$ 0
Service Based Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compenation by share based payment arrangement number of options granted		0	394,213

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Disclosure of Commitement and Contingencies [Line Items]
Lessee operating lease year of expiry			2029		2029
Operating lease agreement option to renewal term	5 years		5 years		5 years
Tenant improvement allowance maximum	$ 14.7		$ 14.7		$ 14.7
Letter of Credit for Security Deposit	2.1		2.1		2.1
Rent expense operating lease	$ 1.8	$ 2.5	$ 5.5	$ 6	7.9	$ 3.5
Tenant receivable					$ 0.4	$ 1.3

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Future Minimum Lease Payments (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
2020 (remaining 3 months)	$ 1,745	$ 6,867
2021	7,053	7,053
2022	7,251	7,251
2023	7,441	7,441
2024	7,679	7,679
Thereafter	35,451	35,451
Future Monimum Lease Payments	$ 66,620	$ 71,742

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		Jan. 31, 2020	Jan. 09, 2020	Dec. 31, 2019	Dec. 31, 2018	Feb. 22, 2018	Sep. 30, 2020	Dec. 31, 2018	Jun. 16, 2020
Temporary Equity [Line Items]
Proceeds from issuance of convertible preferred stock							$ 108,854
Shares issued upon conversion	[1]			14,961,027	14,961,027			14,961,027
Value per share of common stock				$ 0.0001	$ 0.0001		$ 0.0001	$ 0.0001
Original issue price [Member]
Temporary Equity [Line Items]
Shares issued price		$ 5.5914
Preferred Stock, Dividend Rate, Percentage				6.00%
Conversion issue price [Member]
Temporary Equity [Line Items]
Shares issued price		$ 14.2180
Common Stock [Member]
Temporary Equity [Line Items]
Shares issued upon conversion									27,094,085
Series B Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Shares issued during the period						10,974,644		10,974,644
Shares issued price						$ 9.1457
Gross proceeds						$ 100,400
Stock issuance of costs						$ 200		$ 181
Shares issued upon conversion		26,425,664
Series B Convertible Preferred Stock [Member] | Common Stock [Member]
Temporary Equity [Line Items]
Shares issued upon conversion		15,807,081
Series C Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Shares issued during the period		19,936,296	19,936,296				19,936,296
Shares issued price		$ 5.5914	$ 5.5914
Proceeds from issuance of convertible preferred stock		$ 111,500
Gross proceeds		$ 111,500	$ 111,500
Stock issuance of costs			$ 2,600				$ 2,640
Shares issued upon conversion		10,974,644
Series C Convertible Preferred Stock [Member] | Common Stock [Member]
Temporary Equity [Line Items]
Shares issued upon conversion		7,059,406
Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Proceeds from issuance of convertible preferred stock		$ 40,000
Value per share of common stock		$ 8.0496
Series A Preferred Stock [Member]
Temporary Equity [Line Items]
Shares issued upon conversion				8,747,675	8,747,675			8,747,675
Preferred stock conversion basis					1.7663-for-one basis
Series A Preferred Stock [Member] | Original issue price [Member]
Temporary Equity [Line Items]
Shares issued price		$ 1
Series A Preferred Stock [Member] | Conversion issue price [Member]
Temporary Equity [Line Items]
Percentage on conversion ratio		1.7663%
Series B Preferred Stock [Member]
Temporary Equity [Line Items]
Shares issued upon conversion	[1]			6,213,352	6,213,352			6,213,352
Percentage on conversion ratio		57.00%
Preferred stock conversion basis				1.7663-for-one basis
Series B Preferred Stock [Member] | Original issue price [Member]
Temporary Equity [Line Items]
Shares issued price		$ 9.1457
Series B Preferred Stock [Member] | Conversion issue price [Member]
Temporary Equity [Line Items]
Percentage on conversion ratio		16.154%
Series C Preferred Stock [Member]
Temporary Equity [Line Items]
Percentage on conversion ratio		57.00%
Preferred stock conversion basis		1.7663-for-one basis
Series C Preferred Stock [Member] | Original issue price [Member]
Temporary Equity [Line Items]
Shares issued price		$ 5.5914
Series C Preferred Stock [Member] | Conversion issue price [Member]
Temporary Equity [Line Items]
Shares issued price		$ 9.8761
[1]	Upon issuance and sale of the Series C in January 2020 (see Note 13), the 10,974,644 outstanding shares of Series B became convertible into 7,059,406 shares of common stock and the 26,425,664 outstanding shares of Preferred Stock became convertible into 15,807,081 shares of common stock.

Convertible Preferred Stock - S

Convertible Preferred Stock - Summary of Preferred Stock (Detail) - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Preferred Stock [Line Items]
Preferred stock, shares authorized		0	26,425,664	26,425,664
Preferred stock issued and outstanding			26,425,664	26,425,664
Carrying value			$ 115,593	$ 115,593
Liquidation preference			$ 115,822	$ 115,822
Common stock issuable upon conversion	[1]		14,961,027	14,961,027
Series A Preferred Stock [Member]
Schedule Of Preferred Stock [Line Items]
Preferred stock, shares authorized			15,451,020	15,451,020
Preferred stock issued and outstanding			15,451,020	15,451,020
Carrying value			$ 15,404	$ 15,404
Liquidation preference			$ 15,451	$ 15,451
Common stock issuable upon conversion			8,747,675	8,747,675
Series B Preferred Stock [Member]
Schedule Of Preferred Stock [Line Items]
Preferred stock, shares authorized			10,974,644	10,974,644
Preferred stock issued and outstanding			10,974,644	10,974,644
Carrying value			$ 100,189	$ 100,189
Liquidation preference			$ 100,371	$ 100,371
Common stock issuable upon conversion	[1]		6,213,352	6,213,352
[1]	Upon issuance and sale of the Series C in January 2020 (see Note 13), the 10,974,644 outstanding shares of Series B became convertible into 7,059,406 shares of common stock and the 26,425,664 outstanding shares of Preferred Stock became convertible into 15,807,081 shares of common stock.

Convertible Preferred Stock -_2

Convertible Preferred Stock - Summary of Preferred Stock (Parenthetical) (Detail) - shares		Jun. 16, 2020	Jan. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Preferred Stock [Line Items]
No of shares Converted	[1]			14,961,027	14,961,027
Common Stock [Member]
Schedule Of Preferred Stock [Line Items]
No of shares Converted		27,094,085
Series C Convertible Preferred Stock [Member]
Schedule Of Preferred Stock [Line Items]
No of shares Converted			10,974,644
Series C Convertible Preferred Stock [Member] | Common Stock [Member]
Schedule Of Preferred Stock [Line Items]
No of shares Converted			7,059,406
Series B Convertible Preferred Stock [Member]
Schedule Of Preferred Stock [Line Items]
No of shares Converted			26,425,664
Series B Convertible Preferred Stock [Member] | Common Stock [Member]
Schedule Of Preferred Stock [Line Items]
No of shares Converted			15,807,081
[1]	Upon issuance and sale of the Series C in January 2020 (see Note 13), the 10,974,644 outstanding shares of Series B became convertible into 7,059,406 shares of common stock and the 26,425,664 outstanding shares of Preferred Stock became convertible into 15,807,081 shares of common stock.

Net Loss per Share - Summary of

Net Loss per Share - Summary of Potential Dilutive Securities, Presented Based on Amounts Outstanding (Detail) - shares	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive net loss per share	6,538,435	20,102,734	20,040,099	20,565,557
Convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive net loss per share		14,961,027	14,961,027	14,961,027
Unvested restricted common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive net loss per share	956,718	1,970,209	1,705,188	2,819,949
Stock option to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive net loss per share	5,581,717	3,171,498	3,373,884	2,784,581

Subsequent events - Additional

Subsequent events - Additional Information (Detail) $ / shares in Units, $ in Millions		Jun. 05, 2020	Mar. 31, 2020$ / sharesshares	Mar. 05, 2020$ / sharesshares	Jan. 31, 2020USD ($)$ / sharesshares	Jan. 09, 2020USD ($)$ / sharesshares	Feb. 22, 2018USD ($)$ / sharesshares	Sep. 30, 2020shares	Dec. 31, 2019$ / sharesshares	Dec. 31, 2018shares	Jun. 16, 2020shares
Subsequent Event [Line Items]
Convertible preferred stock, shares authorized								0	26,425,664	26,425,664
Convertible Preferred Stock Conversion Price | $ / shares									$ 16.1540
Temporary Equity, Shares Outstanding								0	26,425,664	26,425,664
Shares issued upon conversion	[1]								14,961,027	14,961,027
Share based compenation by share based payment arrangement number of options granted									1,200,954
Share based compenation by share based payment arrangement number of options granted weighted average exercise price per share | $ / shares									$ 5.92
Common Stock [Member]
Subsequent Event [Line Items]
Shares issued upon conversion											27,094,085
Common Stock [Member] | 2017 Stock Incentive Plan
Subsequent Event [Line Items]
Share based compensation by share based payment arrangement number of shares authorized for issuance					10,275,717				8,407,405
Subsequent Event [Member]
Subsequent Event [Line Items]
Convertible preferred stock, shares authorized					46,361,960				26,425,664
Convertible Preferred Stock Conversion Price | $ / shares					$ 14.2180
Reverse Stock Split description		one-for-1.7663 reverse stock split
Stock Split Conversion Ratio		1.7663
Subsequent Event [Member] | 2017 Stock Incentive Plan | Service Based Options [Member]
Subsequent Event [Line Items]
Share based compenation by share based payment arrangement number of options granted			63,834	947,411
Share based compenation by share based payment arrangement number of options granted weighted average exercise price per share | $ / shares			$ 5.25	$ 5.16
Weighted average grant date fair value of Unvested restricted common stock, Issued | $ / shares			$ 0.2	$ 3.4
Stock based compensation vesting period service based			4 years	4 years
Subsequent Event [Member] | 2017 Stock Incentive Plan | Performance Based Options [Member]
Subsequent Event [Line Items]
Share based compenation by share based payment arrangement number of options granted				717,198
Share based compenation by share based payment arrangement number of options granted weighted average exercise price per share | $ / shares				$ 5.16
Weighted average grant date fair value of Unvested restricted common stock, Issued | $ / shares				$ 2.3
Subsequent Event [Member] | Common Stock [Member]
Subsequent Event [Line Items]
Shares issued upon conversion					7,059,406
Series B Convertible Preferred Stock [Member]
Subsequent Event [Line Items]
Shares issued during the period							10,974,644			10,974,644
Shares issued price | $ / shares							$ 9.1457
Gross proceeds | $							$ 100.4
Temporary Equity, Shares Outstanding									10,974,644
Shares issued upon conversion					26,425,664
Series B Convertible Preferred Stock [Member] | Common Stock [Member]
Subsequent Event [Line Items]
Shares issued upon conversion					15,807,081
Series C Convertible Preferred Stock [Member]
Subsequent Event [Line Items]
Shares issued during the period					19,936,296	19,936,296		19,936,296
Shares issued price | $ / shares					$ 5.5914	$ 5.5914
Gross proceeds | $					$ 111.5	$ 111.5
Shares issued upon conversion					10,974,644
Series C Convertible Preferred Stock [Member] | Common Stock [Member]
Subsequent Event [Line Items]
Shares issued upon conversion					7,059,406
[1]	Upon issuance and sale of the Series C in January 2020 (see Note 13), the 10,974,644 outstanding shares of Series B became convertible into 7,059,406 shares of common stock and the 26,425,664 outstanding shares of Preferred Stock became convertible into 15,807,081 shares of common stock.