SQZ Biotechnologies (SQZB) 10-Q 10 Nov 21 2021 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Nov. 03, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Entity Registrant Name	SQZ BIOTECHNOLOGIES COMPANY
Entity Central Index Key	0001604477
Entity Incorporation, State or Country Code	DE
Current Fiscal Year End Date	--12-31
Title of 12(b) Security	Common Stock, $0.001 par value per share
Trading Symbol	SQZ
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Shell Company	false
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		28,066,795
Entity Address, State or Province	MA
Entity File Number	001-39662
Entity Tax Identification Number	46-2431115
Entity Address, Address Line One	200 Arsenal Yards Blvd
Entity Address, Address Line Two	Suite 210
Entity Address, City or Town	Watertown
Entity Address, Postal Zip Code	02472
City Area Code	617
Local Phone Number	758-8672

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 164,254	$ 170,357
Accounts receivable		1,892
Prepaid expenses and other current assets	1,913	4,582
Total current assets	166,167	176,831
Property and equipment, net	3,319	3,645
Restricted cash	2,305	2,305
Operating lease right-of-use assets	72,282	48,360
Total assets	244,073	231,141
Current liabilities:
Accounts payable	2,390	3,708
Accrued expenses	5,661	7,358
Current portion of deferred revenue	21,857	25,917
Current portion of operating lease liabilities	9,201	8,210
Total current liabilities	39,109	45,193
Deferred revenue, net of current portion	9,196	19,659
Operating lease liabilities, net of current portion	62,357	38,885
Other liabilities	205	205
Total liabilities	110,867	103,942
Commitments and contingencies (Note 9)
Stockholders’ equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized at September 30, 2021 and December 31, 2020; No shares issued or outstanding.
Common stock, $0.001 par value; 200,000,000 shares authorized at September 30, 2021 and December 31, 2020; 28,064,709 and 24,786,324 shares issued and outstanding at September 30, 2021 and December 31, 2020, respectively.	28	25
Additional paid-in capital	316,866	253,943
Accumulated deficit	(183,688)	(126,769)
Total stockholders’ equity	133,206	127,199
Total liabilities and stockholders’ equity	$ 244,073	$ 231,141

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	28,064,709	24,786,324
Common stock, shares outstanding	28,064,709	24,786,324

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Total revenue	$ 4,755	$ 6,121	$ 14,748	$ 18,511
Revenue, Product and Service [Extensible List]	sqz:CollaborationMember	sqz:CollaborationMember	sqz:CollaborationMember	sqz:CollaborationMember
Operating expenses:
Research and development	$ 20,520	$ 13,910	$ 52,942	$ 37,815
General and administrative	6,691	4,612	18,744	14,139
Total operating expenses	27,211	18,522	71,686	51,954
Loss from operations	(22,456)	(12,401)	(56,938)	(33,443)
Other income (expense):
Interest income	8	56	28	533
Other income (expense), net	(2)	(6)	(9)	(10)
Total other income, net	6	50	19	523
Net loss	$ (22,450)	$ (12,351)	$ (56,919)	$ (32,920)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.80)	$ (7.03)	$ (2.08)	$ (18.87)
Weighted-average common shares outstanding, basic and diluted	28,050,130	1,758,039	27,421,839	1,744,948
Comprehensive loss:
Net loss	$ (22,450)	$ (12,351)	$ (56,919)	$ (32,920)
Other comprehensive income (loss):
Unrealized gains (losses) on marketable securities, net of tax of $0		(48)		(15)
Comprehensive loss	$ (22,450)	$ (12,399)	$ (56,919)	$ (32,935)

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Unrealized gains (losses) on marketable securities, net of tax	$ 0	$ 0	$ 0	$ 0

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Initial Public Offering [Member]	Convertible Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member]Initial Public Offering [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]Initial Public Offering [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2019	$ (73,515)			$ 2		$ 2,701		$ 30	$ (76,248)
Beginning balance, convertible preferred stock (Shares) at Dec. 31, 2019			13,869,027
Beginning balance, convertible preferred stock at Dec. 31, 2019			$ 132,109
Beginning balance (Shares) at Dec. 31, 2019				1,737,388
Issuance of Series D convertible preferred stock, net of issuance costs			$ 42,248
Issuance of Series D convertible preferred stock, net of issuance costs (Shares)			3,035,192
Issuance of common stock upon exercise of stock options	44					44
Issuance of common stock upon exercise of stock options (Shares)				22,974
Stock-based compensation expense	2,247					2,247
Net loss	(32,920)								(32,920)
Unrealized gains on marketable securities, net of tax of $0	(15)							(15)
Ending balance at Sep. 30, 2020	$ (104,159)			$ 2		4,992		15	(109,168)
Ending balance, convertible preferred stock (Shares) at Sep. 30, 2020	16,904,219		16,904,219
Ending balance, convertible preferred stock at Sep. 30, 2020			$ 174,357
Ending balance (Shares) at Sep. 30, 2020				1,760,362
Beginning balance at Jun. 30, 2020	$ (92,566)			$ 2		4,186		63	(96,817)
Beginning balance, convertible preferred stock (Shares) at Jun. 30, 2020			16,904,219
Beginning balance, convertible preferred stock at Jun. 30, 2020			$ 174,357
Beginning balance (Shares) at Jun. 30, 2020				1,756,018
Issuance of common stock upon exercise of stock options	10					10
Issuance of common stock upon exercise of stock options (Shares)				4,344
Stock-based compensation expense	796					796
Net loss	(12,351)								(12,351)
Unrealized gains on marketable securities, net of tax of $0	(48)							(48)
Ending balance at Sep. 30, 2020	$ (104,159)			$ 2		4,992		$ 15	(109,168)
Ending balance, convertible preferred stock (Shares) at Sep. 30, 2020	16,904,219		16,904,219
Ending balance, convertible preferred stock at Sep. 30, 2020			$ 174,357
Ending balance (Shares) at Sep. 30, 2020				1,760,362
Beginning balance at Dec. 31, 2020	$ 127,199			$ 25		253,943			(126,769)
Beginning balance (Shares) at Dec. 31, 2020				24,786,324
Issuance of common stock		$ 55,602			$ 3		$ 55,599
Issuance of common stock (Shares)					3,000,000
Issuance of common stock upon exercise of stock options	$ 1,131					1,131
Issuance of common stock upon exercise of stock options (Shares)	278,445			278,445
Stock-based compensation expense	$ 6,193					6,193
Net loss	(56,919)								(56,919)
Ending balance at Sep. 30, 2021	133,206			$ 28		316,866			(183,688)
Ending balance (Shares) at Sep. 30, 2021				28,064,769
Beginning balance at Jun. 30, 2021	152,704			$ 28		313,914			(161,238)
Beginning balance (Shares) at Jun. 30, 2021				28,031,404
Issuance of common stock upon exercise of stock options	171					171
Issuance of common stock upon exercise of stock options (Shares)				33,365
Stock-based compensation expense	2,781					2,781
Net loss	(22,450)								(22,450)
Ending balance at Sep. 30, 2021	$ 133,206			$ 28		$ 316,866			$ (183,688)
Ending balance (Shares) at Sep. 30, 2021				28,064,769

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Issuance costs		$ 798
Unrealized gains on marketable securities, net of tax	$ 0		$ 0
Common Stock [Member]
Issuance costs		1,106
Common Stock [Member] | Initial Public Offering [Member]
Issuance costs		$ 798
Series D Convertible Preferred Stock [Member]
Issuance costs			$ 43

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities:
Net loss	$ (56,919)	$ (32,920)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation and amortization expense	932	1,011
Amortization of operating lease right-of-use assets	7,384	7,154
Stock-based compensation expense	6,193	2,247
Accretion of discounts on marketable securities		(9)
Loss on termination of operating lease		108
Loss on disposal of equipment	7
Changes in operating assets and liabilities:
Accounts receivable	1,892	18
Prepaid expenses and other current assets	2,669	243
Accounts payable	(507)	(846)
Accrued expenses	(1,402)	(1,269)
Deferred revenue	(14,523)	7,236
Operating lease liabilities	(6,843)	(6,630)
Other liabilities		267
Net cash used in operating activities	(61,117)	(23,912)
Cash flows from investing activities:
Purchases of property and equipment	(613)	(1,054)
Sales and maturities of marketable securities		51,000
Net cash (used in) provided by investing activities	(613)	49,946
Cash flows from financing activities:
Proceeds from follow-on public offering of common stock, net of commissions and underwriting discounts	56,400
Payment of issuance costs	(798)
Proceeds from issuance of convertible preferred stock, net of issuance costs paid in the period		42,248
Payment of initial public offering costs		(290)
Proceeds from exercise of stock options	1,131	44
Net cash provided by financing activities	55,627	41,757
Net increase in cash, cash equivalents and restricted cash	(6,103)	67,791
Cash, cash equivalents and restricted cash at beginning of period	172,662	41,574
Cash, cash equivalents and restricted cash at end of period	166,559	109,365
Supplemental disclosure of non-cash investing and financing activities:
Lease assets obtained in exchange for operating lease liabilities	31,306	17,049
Deferred offering costs included in accrued expenses at end of period		842
Common Stock [Member]
Cash flows from financing activities:
Payment of issuance costs	$ (1,106)
Convertible Preferred Stock [Member]
Cash flows from financing activities:
Payment of issuance costs		$ (245)

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation SQZ Biotechnologies Company (the “Company”) is a clinical-stage biotechnology company developing cell therapies for patients with cancer, infectious diseases and other serious conditions. The Company uses its proprietary technology, Cell Squeeze, to physically squeeze cells through a microfluidic chip, temporarily opening the cell membrane and enabling biologic material of interest, or cargo, to diffuse into the cell. The Company is using Cell Squeeze technology to create multiple cell therapy platforms focused on directing specific immune responses. The Company was incorporated in March 2013 under the laws of the State of Delaware. The Company is subject to a number of risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, the ability to obtain additional financing, protection of proprietary technology, dependence on key personnel, the ability to attract and retain qualified employees, compliance with government regulations, the impact of the COVID-19 coronavirus, and the clinical and commercial success of its product candidates. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s drug development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. On February 17, 2021, the Company completed a follow-on public offering (the “Follow-on Offering”) pursuant to which it issued and sold 3,000,000 shares of its common stock. The aggregate net proceeds received by the Company from the Follow-on Offering were approximately $ 56.4 million, after deducting underwriting discounts and commissions, but before deducting offering costs payable by the Company, which were approximately $ 0.8 million. 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 primarily with proceeds from sales of convertible preferred stock, payments received in connection with collaboration agreements, proceeds from borrowings under a convertible promissory note, which converted into shares of convertible preferred stock, and, most recently, with proceeds from its initial public offering, (“IPO”) and the Follow-on Offering. The Company has incurred recurring losses since inception, including net losses of $ 56.9 million for the nine months ended September 30, 2021. As of September 30, 2021, the Company had an accumulated deficit of $ 183.7 million. The Company expects to continue to generate operating losses for the foreseeable future. As of 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 from the issuance date of the interim condensed consolidated financial statements. Impact of the COVID-19 Coronavirus In December 2019, a novel strain of coronavirus, which causes the disease known as COVID-19, was reported to have surfaced in Wuhan, China. Since then, COVID-19 coronavirus has spread globally. In March 2020, the World Health Organization declared the COVID-19 outbreak a pandemic and the U.S. government-imposed travel restrictions on travel between the United States, Europe and certain other countries. The outbreak and government measures taken in response have had a significant impact, both direct and indirect, on hospitals, businesses and commerce, as worker shortages have occurred, supply chains have been disrupted, and facilities and production have been suspended. The future progression of the pandemic and its effects on the Company’s business and operations are uncertain. The COVID-19 pandemic has impacted and may continue to impact personnel at third-party manufacturing facilities or the availability or cost of materials, which would disrupt the Company’s supply chain. It also has affected and may continue to affect the Company’s ability to enroll patients in and timely complete its ongoing Phase 1 clinical trials of SQZ-PBMC-HPV and SQZ-AAC-HPV and delay the initiation of future clinical trials, disrupt regulatory activities or have other adverse effects on its business and operations. For example, the Company has experienced delays in receiving supplies of raw materials for its preclinical activities due to the impact of COVID-19 on its suppliers’ ability to timely manufacture these materials, and it has experienced an increase in the transportation cost of its product candidates due to the decreased availability of commercial flights. In addition, the Company has experienced delays in opening clinical trial sites and sites that are open may also have challenges enrolling patients due to the COVID-19 pandemic. Further, staff shortages, including staff that are required to conduct certain testing, such as biopsies, at the Company’s clinical sites or at third-party vendors have resulted in delays in site initiations and in such tests not being properly or timely performed or being delayed. The pandemic has already caused significant disruptions in the financial markets, and may continue to cause such disruptions, which could impact the Company’s ability to raise additional funds to support its operations. Moreover, the pandemic has significantly impacted economies worldwide and could result in adverse effects on the Company’s business and operations. The Company is monitoring the potential impact of the COVID-19 pandemic on its business and financial statements. To date, the Company has not incurred impairment losses in the carrying values of its assets as a result of the pandemic and it is not aware of any specific related event or circumstance that would require it to revise its estimates reflected in these interim condensed consolidated financial statements. The extent to which the COVID-19 pandemic will directly or indirectly impact the Company’s business, results of operations, financial condition and liquidity, including planned and future clinical trials and research and development costs, will depend on future developments that are highly uncertain, including as a result of new information that may emerge concerning COVID-19, the actions taken to contain or treat it, and the duration and intensity of the related effects. Basis of Presentation The Company’s consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly owned subsidiary, SQZ Biotechnologies Security Corporation. All intercompany accounts and transactions have been eliminated in consolidation.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Unaudited Interim Financial Information The accompanying unaudited consolidated financial statements as of September 30, 2021 and for the three and nine months ended September 30, 2021 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. The accompanying consolidated balance sheet as of December 31, 2020 was derived from audited financial statements but does not include all disclosures required by GAAP. Certain information and footnote disclosures normally included in the financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. These interim condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2020 included in the Company’s Form 10-K filed with the SEC, on March 18, 2021. In the opinion of management, all adjustments, consisting only of normal recurring adjustments necessary for a fair statement of the Company’s consolidated financial position as of September 30, 2021, the consolidated results of operations for the three and nine months ended September 30, 2021 and 2020, and the consolidated cash flows for nine months ended September 30, 2021 and 2020 have been made. The Company’s consolidated results of operations for the three and nine months ended September 30, 2021 are not necessarily indicative of the results of operations that may be expected for the full year or any other subsequent interim period. 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 consolidated financial statements, and the reported amounts of revenue and expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition, the accrual of research and development expenses, the valuation of common stock and the valuation of stock-based awards. 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, judgments and methodologies as there are changes in circumstances, facts and experience. Actual results may differ from those estimates or assumptions. Segment Information The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company is developing methods of engineering cell function and therapies for the treatment of patients across a range of indications. The Company has determined that its chief operating decision maker is its Chief Executive Officer. The Company’s chief operating decision maker reviews the Company’s financial information on a consolidated basis for purposes of allocating resources and assessing financial performance. Recently Issued Accounting Pronouncements 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 will adopt the new or revised standard at the time nonpublic companies adopt the new or revised standard and will 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. The Company may choose to early adopt any new or revised accounting standards whenever such early adoption is permitted for nonpublic companies. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses ( Topic 326 ): Measurement of Credit Losses on Financial Instruments (“ASU 2016-13”), which requires the measurement and recognition of expected credit losses for financial assets held at amortized cost. ASU 2016-13 replaces the existing incurred loss impairment model with an expected loss model. It also eliminates the concept of other-than-temporary impairment and requires credit losses related to available-for-sale debt securities to be recorded through an allowance for credit losses rather than as a reduction in the amortized cost basis of the securities. These changes will result in the earlier recognition of credit losses, if any. In May 2019, the FASB issued ASU No. 2019-05, Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief (“ASU 2019-05”) , which provides additional implementation guidance on the previously issued ASU 2016-13. For public entities, this guidance is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. For nonpublic entities, this guidance is effective for fiscal years beginning after December 15, 2022. Early adoption is permitted as of the fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. The Company is currently evaluating the impact that the adoption of ASU 2016-13 and ASU 2019-05 will have on its consolidated financial statements. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 (“ASU 2018-18”). ASU 2018-18 makes targeted improvements to GAAP for collaborative arrangements, including (i) clarification that certain transactions between collaborative arrangement participants should be accounted for as revenue under ASC 606 when the collaborative arrangement participant is a customer in the context of a unit of account, (ii) adding unit-of-account guidance in ASC 808, Collaborative Arrangements , to align with the guidance in ASC 606 and (iii) a requirement that in a transaction with a collaborative arrangement participant that is not directly related to sales to third parties, presenting the transaction together with revenue recognized under ASC 606 is precluded if the collaborative arrangement participant is not a customer. For public entities, this guidance is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. For nonpublic entities, this guidance is effective for fiscal years beginning after December 15, 2020. Early adoption is permitted. The Company adopted ASU 2018-18 as of January 1, 2021, and the standard did not have a material impact on its consolidated financial statements. In December 2019, the FASB issued ASU No. 2019-12, Simplifying the Accounting for Income Taxes (Topic 740) (“ASU 2019-12”), which simplifies the accounting for income taxes by eliminating certain exceptions, including the approach for intraperiod tax allocation, the accounting for income taxes in an interim period, hybrid taxes and the recognition of deferred tax liabilities for outside basis differences. It also clarifies and simplifies other aspects of the accounting for income taxes. For public entities, this guidance is effective for fiscal years beginning after December 15, 2020, including interim periods within those fiscal years. For nonpublic entities, this guidance is effective for fiscal years beginning after December 15, 2021. Early adoption is permitted in interim or annual periods with any adjustments reflected as of the beginning of the fiscal year of adoption. Additionally, entities that elect early adoption must adopt all changes as a result of ASU 2019-12. The Company is currently evaluating the impact that the adoption of ASU 2019-12 will have on its consolidated financial statements.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The following tables present the Company’s fair value hierarchy for its assets and liabilities that are measured at fair value on a recurring basis (in thousands): FAIR VALUE MEASUREMENTS AT LEVEL 1 LEVEL 2 LEVEL 3 TOTAL Assets: Cash equivalents: Money market funds $ 163,908 $ — $ — $ 163,908 $ 163,908 $ — $ — $ 163,908 FAIR VALUE MEASUREMENTS AT LEVEL 1 LEVEL 2 LEVEL 3 TOTAL Assets: Cash equivalents: Money market funds $ 170,097 $ — $ — $ 170,097 $ 170,097 $ — $ — $ 170,097 Money market funds were valued by the Company based on quoted market prices, which represent a Level 1 measurement within the fair value hierarchy. There were no changes to the valuation methods during the nine months ended September 30, 2021 The Company evaluates transfers between levels at the end of each reporting period. There were no transfers between Level 1 or Level 2 during the nine months ended September 30, 2021 .

Property and Equipment, Net

Property and Equipment, Net	9 Months Ended
	Sep. 30, 2021
Property Plant And Equipment [Abstract]
Property and Equipment, Net	4. Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): SEPTEMBER 30, DECEMBER 31, 2021 2020 Machinery and equipment $ 6,660 $ 6,139 Leasehold improvements 579 579 Furniture and fixtures $ 318 459 $ 7,557 $ 7,177 Less: Accumulated depreciation and amortization ( 4,238 ) ( 3,532 ) $ 3,319 $ 3,645 Depreciation and amortization expense was $ 0.3 million for each of the three months ended September 30, 2021 and 2020. Depreciation and amortization expense for the nine months ended September 30, 2021 and 2020 was $ 0.9 million and $ 1.0 million, respectively. In February 2020, as a result of the termination of the 2016 Lease (see Note 10), the Company removed from the consolidated balance sheet leasehold improvements with a cost of $ 2.7 million and accumulated depreciation related to those leasehold improvements of $ 1.3 million. The resulting $ 1.4 million loss was recognized by the Company as a component of the $ 0.1 million net loss on termination for the nine months ended September 30, 2020.

Accrued Expenses

Accrued Expenses	9 Months Ended
	Sep. 30, 2021
Payables And Accruals [Abstract]
Accrued Expenses	5. Accrued Expenses Accrued expenses consisted of the following (in thousands): SEPTEMBER 30, DECEMBER 31, 2021 2020 Accrued external research, development and manufacturing costs $ 1,661 $ 3,085 Accrued employee compensation and benefits 2,512 2,682 Accrued licensing fees (Note 9) 786 743 Other 702 848 $ 5,661 $ 7,358

Preferred Stock

Preferred Stock	9 Months Ended
	Sep. 30, 2021
Temporary Equity [Abstract]
Preferred Stock	6. Preferred Stock In January and February 2020, the Company issued and sold an aggregate of 1,094,247 shares of Series D Preferred Stock at a price of $ 13.9365 per share for gross proceeds of $ 15.2 million. In May and June 2020, the Company issued and sold an additional 1,940,945 shares of Preferred Stock at a price of $ 13.9365 per share for gross proceeds of $ 27.0 million. All of the 16,904,219 shares of Preferred Stock outstanding as of September 30, 2020 as shown in the Condensed Consolidated Statement of Convertible Preferred Stock and Stockholders’ Equity (Deficit) automatically converted into a total of 17,800,084 shares of common stock upon the closing of the IPO in November 2020.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	7. Stock-Based Compensation On October 20, 2020, the Company’s board of directors adopted, and on October 22, 2020 its stockholders approved, the 2020 Incentive Award Plan (the “2020 Plan”), which became effective the day prior to the first public trading date of the Company’s common stock. Following the effectiveness of the 2020 Plan, no further awards are made under the Company’s previous 2014 Stock Incentive Plan (the “2014 Plan”). The 2020 Plan provides for the grant of incentive stock options, non-qualified stock options, stock appreciation rights, restricted stock awards, restricted stock units, and other stock-based awards. The number of shares reserved for issuance under the 2020 Plan was initially equal to 2,690,415 and is subject to an annual increase on the first day of each calendar year. The initial increase began on January 1, 2021 and ends on and includes January 1, 2030, equal to the lesser of (i) 5 % of the aggregate number of shares of common stock outstanding on the final day of the immediately preceding calendar year and (ii) such smaller number of shares of common stock as is determined by the board of directors. The shares of common stock underlying any awards that are forfeited, cancelled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, repurchased or are otherwise terminated by the Company under the 2020 Plan or following the effective date of the 2020 Plan, under the 2014 Plan are added back to the shares of common stock available for issuance under the 2020 Plan. As of September 30, 2021 and December 31, 2020, there were 2,552,357 and 2,079,230 shares available, respectively, for future issuance under the 2020 Plan. On October 20, 2020, the Company’s board of directors adopted, and on October 22, 2020 its stockholders approved, the 2020 Employee Stock Purchase Plan (the ‘‘2020 ESPP’’), which became effective the day prior to the first public trading date of the Company’s common stock. A total of 275,886 shares of common stock was initially reserved for issuance under this plan. The number of shares of common stock that may be issued under the 2020 ESPP automatically increases on the first day of each calendar year. The initial increase began on January 1, 2021 and ends on and includes January 1, 2030, equal to the lesser of (i) 1 % of the shares of common stock outstanding on the final day of the immediately preceding calendar year and (ii) such smaller number of shares of common stock as is determined by the board of directors, provided that not more than 3,724,461 shares of common stock may be issued under the 2020 ESPP. The initial six-month offering period commenced on July 1, 2021 and ends on December 31, 2021. As of September 30, 2021 , no shares had been issued under the 2020 ESPP and there were 523,749 shares available for issuance. Stock Option Valuation The fair value of each option is estimated on the date of grant 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 public 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. 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 option. 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 expected term of stock options granted to non-employees is equal to the contractual term of the option award. The expected dividend yield of 0 % 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 summarizes the Company’s stock option activity since December 31, 2020: NUMBER OF WEIGHTED- WEIGHTED- INTRINSIC (in years) (in thousands) Outstanding at December 31, 2020 4,039,894 $ 7.69 8.41 $ 85,993 Granted 1,218,982 15.79 Exercised ( 278,445 ) 4.06 Forfeited or canceled ( 452,789 ) 10.38 Outstanding at September 30, 2021 4,527,642 $ 9.83 8.16 $ 24,441 Vested and expected to vest at September 30, 2021 4,527,642 $ 9.83 8.16 $ 24,441 Options exercisable at September 30, 2021 1,742,018 $ 5.13 6.99 $ 16,223 Stock-Based Compensation Expense Stock-based compensation expense related to stock options was classified in the consolidated statements of operations as follows (in thousands): THREE MONTHS ENDED NINE MONTHS ENDED 2021 2020 2021 2020 Research and development expenses $ 1,423 $ 296 $ 2,670 $ 811 General and administrative expenses 1,358 500 3,523 1,436 $ 2,781 $ 796 $ 6,193 $ 2,247 In September 2021, the Company modified the terms of stock options previously granted to an executive officer, and due to expire in December 2021. As a result of the modification, the Company recorded an expense of approximately $ 0.9 million to account for the incremental change in the fair value of the stock options before and after the modification, which was recognized as compensation cost within research and development expenses. As of September 30, 2021 , total unrecognized stock-based compensation expense related to unvested stock-based awards was $ 21.3 million, which is expected to be recognized over a weighted-average period of 2.7 years.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2021
Income Tax Disclosure [Abstract]
Income Taxes	8. Income Taxes For the three and nine months ended September 30, 2021 and 2020 , the Company recorded no income tax benefits for the net operating losses incurred or for the research and development tax credits generated in each period, 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.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2021
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	9. Commitments and Contingencies Leases The Company’s commitments under its leases are described in Note 10. License and Supply Agreements License Agreement with Massachusetts Institute of Technology In December 2015, the Company entered into an exclusive patent license agreement with the Massachusetts Institute of Technology (“MIT”) (the “MIT Agreement”). The MIT Agreement replaced a May 2013 exclusive agreement with MIT. Under the MIT Agreement, the Company received an exclusive license under the licensed patent rights to develop, manufacture and commercialize any products related to certain intracellular delivery methods that were developed at MIT. As of September 30, 2021 and December 31, 2020, the Company had liabilities of $ 0.8 million and $ 0.7 million, respectively, included within accrued expenses (see Note 5). During each of the nine months ended September 30, 2021 and 2020, the Company did no t recognize any research and development expense under the sublicense terms of the MIT Agreement. Manufacturing Services Agreements The Company has entered into agreements with a contract manufacturing organization to provide manufacturing services related to its product candidates. As of September 30, 2021 the Company had no non-cancelable payments under these agreements, as amended, other than the amounts included in the current portion of operating lease liabilities on the Company's consolidated balance sheets. 401(k) Plan The Company sponsors a 401(k) defined contribution benefit plan (the “401(k) Plan”), which covers all employees who meet certain eligibility requirements as defined in the 401(k) Plan and allows participants to defer a portion of their annual compensation on a pre-tax basis. Contributions to the 401(k) Plan may be made at the discretion of management. For each of the three months ended September 30, 2021 and 2020, the Company contributed $ 0.1 million to the 401(k) Plan. For each of the nine months ended September 30, 2021 and 2020, the Company contributed $ 0.3 million to the 401(k) Plan. Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to its 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 its executive officers and members of its board of directors 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. 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 indemnification agreements and is not currently aware of any indemnification claims. Legal Proceedings The Company is not currently party to any material legal proceedings. At each reporting date, the Company evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses as incurred the costs related to such legal proceedings.

Leases

Leases	9 Months Ended
	Sep. 30, 2021
Leases [Abstract]
Leases	10. Leases As of September 30, 2021, the Company leases its office and laboratory facilities under a non-cancelable operating lease entered into in December 2018, which included lease incentives, payment escalations and rent holidays. The Company had not entered into any financing leases or any short-term operating leases as of September 30, 2021 and December 31, 2020. 2018 Lease In December 2018, the Company entered into a lease for office and laboratory space in Watertown, Massachusetts (the “2018 Lease”). The 2018 Lease term commenced in December 2019 and expires in November 2029 . Under the 2018 Lease, the Company has one five-year option to extend the term of the lease. The initial annual base rent was $ 3.8 million upon entering into the lease, with such base rent increasing during the initial term by 3 % annually on the anniversary of the commencement date. The Company is obligated to pay its portion of real estate taxes and costs related to the premises, including costs of operations, maintenance, repair, replacement and management of the new leased premises. In connection with the lease, the Company maintains a letter of credit for the benefit of the landlord in the amount of $ 2.3 million, for which the Company is required to maintain a separate cash balance of the same amount. The 2018 Lease Agreement includes a landlord-provided tenant improvement allowance of $ 9.8 million that was applied to the costs of the construction of leasehold improvements. 2016 Lease In September 2016, the Company entered into a lease for office and laboratory space in Watertown, Massachusetts (the “2016 Lease”). The 2016 Lease was set to expire in September 2023; however, in February 2020, the Company and the landlord jointly terminated the 2016 Lease. Accordingly, as of February 2020, the Company had no further obligations under the 2016 Lease. As a result of this termination, the Company removed from the consolidated balance sheet the associated operating lease right-of-use asset of $ 2.1 million, leasehold improvements with a net carrying value of $ 1.4 million (see Note 4) and operating lease liabilities of $ 3.4 million. The Company therefore recognized a net loss on termination of the 2016 Lease of $ 0.1 million in the consolidated statement of operations and comprehensive loss for the nine months ended September 30, 2020. Embedded Lease The Company evaluated its vendor contracts to identify embedded leases, if any, and noted that an agreement entered into in April 2019 with a contract manufacturing supplier constituted a lease under ASC 842 because the Company has the right to substantially all of the economic benefits from the use of the asset and can direct the use of the asset. The embedded lease commenced in September 2019 and had an initial term of two years . In September 2020, the Company amended the lease to extend the term of the lease by an additional two years with the agreement to end in August 2022 . In September 2021, the Company amended the terms of its agreement to allow for an increase in manufacturing runs, and to extend the term of the agreement through December 2026. This resulted in an increase in the estimated future payments to be made by the Company to the contract manufacturing supplier. The Company determined that the amendment constituted a modification of the existing agreement under ASC 842, rather than a separate contract. Upon the modification in September 2021, the Company recorded increases in right-of-use assets and operating lease liabilities in equal amounts of $ 31.3 million. Right-of-use assets under operating leases at September 30, 2021 and December 31, 2020 totaled $ 72.3 million and $ 48.4 million, respectively. The leases do not include any restrictions or covenants that had to be accounted for under applicable lease guidance. Lease Portfolio The components of lease cost and other information for the Company’s lease portfolio were as follows (in thousands, except term and discount rate amounts): THREE MONTHS ENDED NINE MONTHS ENDED 2021 2020 2021 2020 Lease cost: Operating lease cost $ 3,430 $ 3,231 $ 9,965 $ 9,345 Variable lease cost 425 306 1,428 908 Short-term lease cost — — — 21 $ 3,855 $ 3,537 $ 11,393 $ 10,274 SEPTEMBER 30, DECEMBER 31, Other information: Weighted-average remaining lease term (in years) 6.4 6.7 Weighted-average discount rate 7.6 % 7.2 % Supplemental cash flow information related to the Company’s operating leases was as follows (in thousands): NINE MONTHS ENDED 2021 2020 Cash paid for amounts included in the measurement of operating lease Operating cash flows from operating leases $ 9,424 $ 8,820

License and Collaboration Agree

License and Collaboration Agreements	9 Months Ended
	Sep. 30, 2021
Research And Development [Abstract]
License and Collaboration Agreements	11. License and Collaboration Agreements 2017 License and Collaboration Agreement with Roche In April 2017, the Company entered into a license and collaboration agreement with Roche (the “2017 Roche Agreement”) to allow Roche to use the Company’s Cell Squeeze technology to enable gene editing of immune cells to discover new targets in cancer immunotherapy. The 2017 Roche Agreement includes several licenses granted by Roche to the Company and by the Company to Roche in order to conduct a specified research program in accordance with a specified research plan. The 2017 Roche Agreement has a term that ends upon the earlier to occur of (i) the completion of all work under the research plan or (ii) two years after the effective date of the agreement. The collaboration term is subject to Roche’s right to extend the collaboration term for up to two additional one-year periods. Roche has the right to terminate the agreement, in whole or on a workstream-by-workstream basis, upon a specified amount of notice to the Company. The Company or Roche may terminate the agreement if the other party fails to cure its material breach within a specified period after receiving notice of such breach. Under the agreement, the Company received an upfront payment of $ 5.0 million as a technology access fee and is entitled to (i) payments of up to $ 1.0 million, in two tranches of $ 0.5 million, as reimbursement for the Company’s research costs; (ii) milestone payments of up to $ 7.0 million upon the achievement of specified development milestones; and (iii) annual maintenance fees ranging from $ 0.5 million to $ 0.9 million for each year following the fifth anniversary of the effective date, subject to specified prepayment discounts. The Company assessed its accounting for the 2017 Roche Agreement under ASC 606 as the transactions underlying the agreement were deemed to be transactions with a customer. The Company identified the following promises under the 2017 Roche Agreement: (i) a non-exclusive license granted to Roche to perform research related to and use of the Company’s Cell Squeeze technology for gene editing of immune cells; (ii) specified research and development services related to gene editing of immune cells through the research term; (iii) manufacturing activities to support the specified research plan; and (iv) participation on a joint research committee (“JRC”). The annual maintenance fees described above were determined by the Company to be optional renewal payments. The Company concluded that each of the promises under the agreement was not distinct from the other promises in the arrangement. The research license was determined to not be distinct from the research and manufacturing activities primarily as a result of Roche being unable to benefit on its own or with other resources reasonably available in the marketplace because the license to the Company’s intellectual property requires significant specialized capabilities in order to be further developed, the research services necessary to develop the product are highly specialized, and the Company’s proprietary Cell Squeeze technology is a key capability of that development. The research and manufacturing services were determined not to be distinct because the promise under the agreement is to complete research and development, inclusive of the manufacturing. In addition, the Company determined that the impact of participation on the JRC was insignificant and had an immaterial impact on the accounting model. As such, the Company concluded that the first three promises should be combined into a single performance obligation. Based on these assessments, the Company identified one distinct performance obligation at the outset of the 2017 Roche Agreement. The Company recognizes revenue associated with the performance obligation as the research, development and manufacturing services are provided using an input method, based on the cumulative costs incurred compared to the total estimated costs expected to be incurred to satisfy the performance obligation. The transfer of control to the customer occurs over the time period that the research and development services are to be provided by the Company, and this cost-to-cost method is, in management’s judgment, the best measure of progress towards satisfying the performance obligation. The amounts received from Roche that have not yet been recognized as revenue are deferred as a contract liability in the Company’s consolidated balance sheet and will be recognized over the remaining research and development period until the performance obligation is satisfied. During the three and nine months ended September 30, 2021, the total costs expected to be incurred to satisfy the performance obligation under the 2017 Roche Agreement decreased by $ 0.1 million and $ 0.4 million, respectively. During the three and nine months ended September 30, 2020 , there were no significant changes in the total estimated costs expected to be incurred to satisfy the performance obligation. The Company recognized revenue of $ 0.3 million and $ 0.1 million, respectively, during the three months ended September 30, 2021 and 2020 under the 2017 Roche Agreement. The Company recognized revenue of $ 1.0 million and $ 0.4 million, respectively, during the nine months ended September 30, 2021 and 2020 under this agreement. As of September 30, 2021, the Company recorded as a contract liability deferred revenue related to the 2017 Roche Agreement of $ 0.2 million, all of which was a current liability. As of September 30, 2021, the research and development services related to the performance obligation were expected to be performed over a remaining period of approximately nine months . 2018 License and Collaboration Agreement with Roche In October 2018, the Company entered into a license and collaboration agreement with Roche (the “2018 Roche Agreement”) to jointly develop certain products based on mononuclear antigen presenting cells (“APCs”), including human papilloma virus (“HPV”), using the SQZ APC platform for the treatment of oncology indications. The Company granted Roche a non-exclusive license to its intellectual property, and Roche granted the Company a non-exclusive license to its and its affiliates’ intellectual property for the purpose of performing research activities. In connection with this agreement, the parties terminated an earlier agreement. The 2018 Roche Agreement has a term that extends until all royalty, profit-share and other payment obligations expire or have been satisfied. Roche has the right to terminate the 2018 Roche Agreement, in whole or on a product-by-product basis, upon a specified amount of notice to the Company. The Company or Roche may terminate the agreement if the other party fails to cure its material breach within a specified period after receiving notice of such breach. Under the 2018 Roche Agreement, Roche was granted option rights to obtain an exclusive license to develop APC products or products derived from the collaboration programs on a product-by-product basis. These option rights are exercisable upon the achievement of clinical Phase 1 proof of concept and expire, if unexercised, as of a date specified in the agreement. In addition, Roche was granted an option right to obtain an exclusive license to develop a Tumor Cell Lysate (“TCL”) product. This option right is exercisable upon the achievement of clinical proof of concept and expires, if unexercised, as of a date specified in the agreement. For each of the APC products and TCL product, once Roche exercises its option and pays a specified incremental amount ranging from $ 15.0 million to $ 50.0 million for APC products and of $ 100.0 million for the TCL product, Roche will receive worldwide, exclusive commercialization rights for the licensed products, subject to the Company’s alternating option to retain U.S. APC commercialization rights. The Company will retain worldwide commercialization rights to any APC products or the TCL product for which Roche elects not to exercise its applicable option. For the first APC product that Roche exercises its option, Roche will receive worldwide, exclusive commercialization rights for the licensed product. On a product-by-product basis for the APC products, after the first product option is exercised by Roche and for every other product for which Roche exercises its option, the Company will retain an option to obtain the exclusive commercialization rights in the United States. Upon exercise of the TCL option by Roche, (i) the Company will be entitled to receive the aforementioned milestone payment of $ 100.0 million and (ii) profits from the TCL product will be shared equally by the Company and Roche. Through September 30, 2021, Roche had not exercised any of its options under the 2018 Roche Agreement. Under the 2018 Roche Agreement, the Company received an upfront payment of $ 45.0 million and is eligible to receive (i) reimbursement of a mid-double-digit percentage of its development costs; (ii) aggregate milestone payments on a product-by-product basis of up to $ 1.6 billion upon the achievement of specified milestones, consisting of up to $ 217.0 million of development milestone payments, up to $ 240.0 million of regulatory milestone payments and up to $ 1.2 billion of sales milestone payments; and (iii) tiered royalties on annual net sales of APC and TCL products licensed under the agreement, as described below. The Company received the upfront payment of $ 45.0 million in October 2018 upon execution of the agreement. In addition, during the second quarter of 2019, the Company received a payment of $ 10.0 million following the achievement of the first development milestone under the 2018 Roche Agreement related to submission by the Company of preclinical data to the U.S. Food and Drug Administration (“FDA”), and during the first quarter of 2020, the Company received a payment of $ 20.0 million following the achievement of the second development milestone under the 2018 Roche Agreement related to first-patient dosing in a Phase 1 clinical trial. Roche will pay tiered royalties based on annual net sales of APC and TCL products. If Roche exercises its option to obtain a license to commercialize an APC product, Roche will pay the Company tiered royalties on annual net sales of that licensed product at rates ranging from a mid-single-digit percentage to a mid-teens percentage, depending on net sales of the product. If the Company exercises its option to obtain a license to commercialize an APC product in the United States, it will pay Roche tiered royalties on annual net sales of that licensed product at rates ranging from a mid-single-digit percentage to a mid-teens percentage, depending on net sales of the product in the United States. For APC products selected by Roche, rather than mutually, Roche will pay the Company royalties on annual net sales of that licensed product at rates ranging from a mid-single-digit percentage to a high single-digit percentage, depending on net sales of the product. For APC products that are selected mutually and for which the Company has not exercised its option to commercialize the product in the United States, Roche will pay the Company tiered royalties on annual net sales of that licensed product at a rate ranging from a high single-digit percentage to a mid-teens percentage, depending on net sales of the product. For TCL products, Roche will pay the Company tiered royalties on the aggregate net sales of all TCL products at rates ranging from either a mid-single digit percentage to a percentage in the low twenties, with the caveat that the rates for sales in the United States may instead range from a low-teens percentage to a percentage in the mid-twenties, depending on whether and when the Company opts out of sharing certain profits and costs of commercializing the TCL product in the United States with Roche. The Company identified three performance obligations at the outset of the 2018 Roche Agreement: (1) the license to the Company’s intellectual property, the research and development activities related to HPV through Phase 1 clinical trials under a specified research plan, and the manufacturing of the Company’s SQZ APC platform and equipment in order to support the HPV research plan (the “first performance obligation”); (2) the license to the Company’s intellectual property and the research and development activities on next-generation APCs (the “second performance obligation”); and (3) the license to the Company’s intellectual property and the research and development activities on TCL (the “third performance obligation”). During the second quarter of 2019, the Company received a payment of $ 10.0 million following the achievement of the first development milestone under the 2018 Roche Agreement related to submission by the Company of preclinical data to the U.S. Food and Drug Administration, or FDA. During the first quarter of 2020, the Company received a payment of $ 20.0 million following the achievement of the second development milestone under the 2018 Roche Agreement related to first-patient dosing in a Phase 1 clinical trial. These milestones were added to the transaction price in the period that it was “most likely” and that it was probable that a significant reversal in the amount of cumulative revenue recognized would not occur. During the fourth quarter of 2019, the Company evaluated its overall program priorities and determined that it would continue to focus its resources on progressing the specified APC programs related to the 2018 Roche Agreement as well as its Activating Antigen Carriers (“AAC”) and Tolerizing Antigen Carriers (“TAC”) platforms. As a result of its continuing focus on these specific programs, the Company reduced the level of priority of the TCL research activities under the 2018 Roche Agreement and expects to perform such TCL research activities over a longer time period than as originally expected under the specified research plan of the agreement. Since the fourth quarter of 2019, the Company has classified $ 9.2 million as non-current deferred revenue, which will remain unrecognized as revenue until TCL research activities resume or the 2018 Roche Agreement is modified by the Company and Roche. The Company separately recognizes revenue associated with each of the three performance obligations as the research, development and manufacturing services are provided using an input method, based on the cumulative costs incurred compared to the total estimated costs expected to be incurred to satisfy each performance obligation. The transfer of control to the customer occurs over the time period that the research and development services are to be provided by the Company, and this cost-to-cost method is, in management’s judgment, the best measure of progress towards satisfying each performance obligation. The amounts received that have not yet been recognized as revenue are deferred as a contract liability in the Company’s consolidated balance sheet and will be recognized over the remaining research and development period until each performance obligation is satisfied. During the three and nine months ended September 30, 2021, the estimated costs expected to be incurred to satisfy the performance obligations increased by $ 0.4 million. During the three and nine months ended September 30, 2020, there were no significant changes in the total estimated costs expected to be incurred to satisfy the performance obligations under the 2018 Roche Agreement. The Company recognized revenue of $ 4.5 million and $ 6.0 million during the three months ended September 30, 2021 and 2020, respectively, under this agreement. The Company recognized revenue of $ 13.6 million and $ 18.1 million during the nine months ended September 30, 2021 and 2020, respectively, under the 2018 Roche Agreement. As of September 30, 2021, the Company recorded as a contract liability deferred revenue related to the 2018 Roche Agreement of $ 30.4 million, of which $ 21.2 million was a current liability. As of September 30, 2021, the research and development services related to the performance obligations were expected to be performed over remaining periods ranging from three to nine months . As of December 31, 2020, the Company recorded as a contract liability deferred revenue related to the 2018 Roche Agreement of $ 44.0 million, of which $ 24.7 million was a current liability. As of September 30, 2021 and December 31, 2020, the expected remaining period of performance of the Company’s research and development services related to the third performance obligation was not determinable, and it will not become determinable until TCL research activities resume or the 2018 Roche Agreement is modified by the Company and Roche. Contract Liability The changes in the total contract liability (deferred revenue) balances related to the Company’s license and collaboration agreements were as follows (in thousands): THREE MONTHS ENDED NINE MONTHS ENDED 2021 2020 2021 2020 Balance at beginning of period $ 35,308 $ 51,918 $ 45,201 $ 40,453 Deferral of revenue — 1,891 — 25,746 Other — — 100 — Recognition of deferred revenue ( 4,755 ) ( 6,121 ) ( 14,748 ) ( 18,511 ) Balance at end of period $ 30,553 $ 47,688 $ 30,553 $ 47,688

Net Loss per Share

Net Loss per Share	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Net Loss per Share	12. Net Loss per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts): THREE MONTHS ENDED NINE MONTHS ENDED 2021 2020 2021 2020 Numerator: Net loss $ ( 22,450 ) $ ( 12,351 ) $ ( 56,919 ) $ ( 32,920 ) Denominator: Weighted-average common shares outstanding, basic and 28,050,130 1,758,039 27,421,839 1,744,948 Net loss per share attributable to common stockholders, basic and $ ( 0.80 ) $ ( 7.03 ) $ ( 2.08 ) $ ( 18.87 ) The Company’s potential dilutive securities, which in the current period consist of common stock options and in the previous period included convertible preferred stock, a warrant to purchase common stock and common stock options, have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted-average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. 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 because including them would have had an anti-dilutive effect: THREE MONTHS ENDED NINE MONTHS ENDED 2021 2020 2021 2020 Convertible preferred stock (as converted to common stock) — 17,800,084 — 17,800,084 Warrant to purchase common stock — 2,038 — 2,038 Stock options to purchase common stock 4,527,642 3,744,451 4,527,642 3,744,451 4,527,642 21,546,573 4,527,642 21,546,573

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2021
Subsequent Events [Abstract]
Subsequent Events	13. Subsequent Events In October 2021, an independent panel recommended that the Company’s SQZ-PBMC-HPV-101 clinical trial advance to combination therapy with checkpoint inhibitors. Upon initiation of a combination therapy cohort, the Company is entitled to receive a $ 3.0 million milestone payment from Roche in accordance with the terms of the Accord related to the 2018 Roche Agreement.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Unaudited Interim Financial Information	Unaudited Interim Financial Information The accompanying unaudited consolidated financial statements as of September 30, 2021 and for the three and nine months ended September 30, 2021 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. The accompanying consolidated balance sheet as of December 31, 2020 was derived from audited financial statements but does not include all disclosures required by GAAP. Certain information and footnote disclosures normally included in the financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. These interim condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2020 included in the Company’s Form 10-K filed with the SEC, on March 18, 2021. In the opinion of management, all adjustments, consisting only of normal recurring adjustments necessary for a fair statement of the Company’s consolidated financial position as of September 30, 2021, the consolidated results of operations for the three and nine months ended September 30, 2021 and 2020, and the consolidated cash flows for nine months ended September 30, 2021 and 2020 have been made. The Company’s consolidated results of operations for the three and nine months ended September 30, 2021 are not necessarily indicative of the results of operations that may be expected for the full year or any other subsequent interim period.
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 consolidated financial statements, and the reported amounts of revenue and expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition, the accrual of research and development expenses, the valuation of common stock and the valuation of stock-based awards. 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, judgments and methodologies as there are changes in circumstances, facts and experience. Actual results may differ from those estimates or assumptions.
Segment Information	Segment Information The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company is developing methods of engineering cell function and therapies for the treatment of patients across a range of indications. The Company has determined that its chief operating decision maker is its Chief Executive Officer. The Company’s chief operating decision maker reviews the Company’s financial information on a consolidated basis for purposes of allocating resources and assessing financial performance.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements 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 will adopt the new or revised standard at the time nonpublic companies adopt the new or revised standard and will 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. The Company may choose to early adopt any new or revised accounting standards whenever such early adoption is permitted for nonpublic companies. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses ( Topic 326 ): Measurement of Credit Losses on Financial Instruments (“ASU 2016-13”), which requires the measurement and recognition of expected credit losses for financial assets held at amortized cost. ASU 2016-13 replaces the existing incurred loss impairment model with an expected loss model. It also eliminates the concept of other-than-temporary impairment and requires credit losses related to available-for-sale debt securities to be recorded through an allowance for credit losses rather than as a reduction in the amortized cost basis of the securities. These changes will result in the earlier recognition of credit losses, if any. In May 2019, the FASB issued ASU No. 2019-05, Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief (“ASU 2019-05”) , which provides additional implementation guidance on the previously issued ASU 2016-13. For public entities, this guidance is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. For nonpublic entities, this guidance is effective for fiscal years beginning after December 15, 2022. Early adoption is permitted as of the fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. The Company is currently evaluating the impact that the adoption of ASU 2016-13 and ASU 2019-05 will have on its consolidated financial statements. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 (“ASU 2018-18”). ASU 2018-18 makes targeted improvements to GAAP for collaborative arrangements, including (i) clarification that certain transactions between collaborative arrangement participants should be accounted for as revenue under ASC 606 when the collaborative arrangement participant is a customer in the context of a unit of account, (ii) adding unit-of-account guidance in ASC 808, Collaborative Arrangements , to align with the guidance in ASC 606 and (iii) a requirement that in a transaction with a collaborative arrangement participant that is not directly related to sales to third parties, presenting the transaction together with revenue recognized under ASC 606 is precluded if the collaborative arrangement participant is not a customer. For public entities, this guidance is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. For nonpublic entities, this guidance is effective for fiscal years beginning after December 15, 2020. Early adoption is permitted. The Company adopted ASU 2018-18 as of January 1, 2021, and the standard did not have a material impact on its consolidated financial statements. In December 2019, the FASB issued ASU No. 2019-12, Simplifying the Accounting for Income Taxes (Topic 740) (“ASU 2019-12”), which simplifies the accounting for income taxes by eliminating certain exceptions, including the approach for intraperiod tax allocation, the accounting for income taxes in an interim period, hybrid taxes and the recognition of deferred tax liabilities for outside basis differences. It also clarifies and simplifies other aspects of the accounting for income taxes. For public entities, this guidance is effective for fiscal years beginning after December 15, 2020, including interim periods within those fiscal years. For nonpublic entities, this guidance is effective for fiscal years beginning after December 15, 2021. Early adoption is permitted in interim or annual periods with any adjustments reflected as of the beginning of the fiscal year of adoption. Additionally, entities that elect early adoption must adopt all changes as a result of ASU 2019-12. The Company is currently evaluating the impact that the adoption of ASU 2019-12 will have on its consolidated financial statements.

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Summary of fair value hierarchy for assets and liabilities	The following tables present the Company’s fair value hierarchy for its assets and liabilities that are measured at fair value on a recurring basis (in thousands): FAIR VALUE MEASUREMENTS AT LEVEL 1 LEVEL 2 LEVEL 3 TOTAL Assets: Cash equivalents: Money market funds $ 163,908 $ — $ — $ 163,908 $ 163,908 $ — $ — $ 163,908 FAIR VALUE MEASUREMENTS AT LEVEL 1 LEVEL 2 LEVEL 3 TOTAL Assets: Cash equivalents: Money market funds $ 170,097 $ — $ — $ 170,097 $ 170,097 $ — $ — $ 170,097

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	9 Months Ended
	Sep. 30, 2021
Property Plant And Equipment Net [Abstract]
Summary of property and equipment, net	Property and equipment, net consisted of the following (in thousands): SEPTEMBER 30, DECEMBER 31, 2021 2020 Machinery and equipment $ 6,660 $ 6,139 Leasehold improvements 579 579 Furniture and fixtures $ 318 459 $ 7,557 $ 7,177 Less: Accumulated depreciation and amortization ( 4,238 ) ( 3,532 ) $ 3,319 $ 3,645

Accrued Expenses (Tables)

Accrued Expenses (Tables)	9 Months Ended
	Sep. 30, 2021
Payables And Accruals [Abstract]
Summary of accrued expenses	Accrued expenses consisted of the following (in thousands): SEPTEMBER 30, DECEMBER 31, 2021 2020 Accrued external research, development and manufacturing costs $ 1,661 $ 3,085 Accrued employee compensation and benefits 2,512 2,682 Accrued licensing fees (Note 9) 786 743 Other 702 848 $ 5,661 $ 7,358

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity since December 31, 2020: NUMBER OF WEIGHTED- WEIGHTED- INTRINSIC (in years) (in thousands) Outstanding at December 31, 2020 4,039,894 $ 7.69 8.41 $ 85,993 Granted 1,218,982 15.79 Exercised ( 278,445 ) 4.06 Forfeited or canceled ( 452,789 ) 10.38 Outstanding at September 30, 2021 4,527,642 $ 9.83 8.16 $ 24,441 Vested and expected to vest at September 30, 2021 4,527,642 $ 9.83 8.16 $ 24,441 Options exercisable at September 30, 2021 1,742,018 $ 5.13 6.99 $ 16,223
Summary of Stock-Based Compensation Expense	Stock-based compensation expense related to stock options was classified in the consolidated statements of operations as follows (in thousands): THREE MONTHS ENDED NINE MONTHS ENDED 2021 2020 2021 2020 Research and development expenses $ 1,423 $ 296 $ 2,670 $ 811 General and administrative expenses 1,358 500 3,523 1,436 $ 2,781 $ 796 $ 6,193 $ 2,247

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2021
Leases [Abstract]
Summary of Components of Lease Cost and Other Information	Lease Portfolio The components of lease cost and other information for the Company’s lease portfolio were as follows (in thousands, except term and discount rate amounts): THREE MONTHS ENDED NINE MONTHS ENDED 2021 2020 2021 2020 Lease cost: Operating lease cost $ 3,430 $ 3,231 $ 9,965 $ 9,345 Variable lease cost 425 306 1,428 908 Short-term lease cost — — — 21 $ 3,855 $ 3,537 $ 11,393 $ 10,274 SEPTEMBER 30, DECEMBER 31, Other information: Weighted-average remaining lease term (in years) 6.4 6.7 Weighted-average discount rate 7.6 % 7.2 %
Summary of Supplementary Cash Flow Information Relating to Operating Leases	Supplemental cash flow information related to the Company’s operating leases was as follows (in thousands): NINE MONTHS ENDED 2021 2020 Cash paid for amounts included in the measurement of operating lease Operating cash flows from operating leases $ 9,424 $ 8,820

License and Collaboration Agr_2

License and Collaboration Agreements (Tables)	9 Months Ended
	Sep. 30, 2021
Research And Development [Abstract]
Summary of Changes in the Total Contract Liability	The changes in the total contract liability (deferred revenue) balances related to the Company’s license and collaboration agreements were as follows (in thousands): THREE MONTHS ENDED NINE MONTHS ENDED 2021 2020 2021 2020 Balance at beginning of period $ 35,308 $ 51,918 $ 45,201 $ 40,453 Deferral of revenue — 1,891 — 25,746 Other — — 100 — Recognition of deferred revenue ( 4,755 ) ( 6,121 ) ( 14,748 ) ( 18,511 ) Balance at end of period $ 30,553 $ 47,688 $ 30,553 $ 47,688

Net Loss per Share (Tables)

Net Loss per Share (Tables)	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Summary of basic and diluted net loss per share attributable to common stockholders	Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts): THREE MONTHS ENDED NINE MONTHS ENDED 2021 2020 2021 2020 Numerator: Net loss $ ( 22,450 ) $ ( 12,351 ) $ ( 56,919 ) $ ( 32,920 ) Denominator: Weighted-average common shares outstanding, basic and 28,050,130 1,758,039 27,421,839 1,744,948 Net loss per share attributable to common stockholders, basic and $ ( 0.80 ) $ ( 7.03 ) $ ( 2.08 ) $ ( 18.87 )
Summary of potentially dilutive shares excluded from the calculation of diluted net loss	The Company’s potential dilutive securities, which in the current period consist of common stock options and in the previous period included convertible preferred stock, a warrant to purchase common stock and common stock options, have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted-average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. 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 because including them would have had an anti-dilutive effect: THREE MONTHS ENDED NINE MONTHS ENDED 2021 2020 2021 2020 Convertible preferred stock (as converted to common stock) — 17,800,084 — 17,800,084 Warrant to purchase common stock — 2,038 — 2,038 Stock options to purchase common stock 4,527,642 3,744,451 4,527,642 3,744,451 4,527,642 21,546,573 4,527,642 21,546,573

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	Feb. 17, 2021	Sep. 30, 2021	Dec. 31, 2020
Income (loss) from continuing operations		$ (56,900)
Accumulated deficit		$ 183,688	$ 126,769
Follow-on Offering [Member]
Issuance of common stock (Shares)	3,000,000
Proceeds from follow on public offer net of underwriting discounts and before payment of offering costs	$ 56,400
Offering costs payable	$ 800

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Fair Value Hierarchy for Assets and Liabilities (Detail) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets:
Assets, fair value disclosure	$ 163,908	$ 170,097
Level 1 [Member]
Assets:
Assets, fair value disclosure	163,908	170,097
Money Market Funds [Member] | Cash equivalents [Member]
Assets:
Assets, fair value disclosure	163,908	170,097
Money Market Funds [Member] | Cash equivalents [Member] | Level 1 [Member]
Assets:
Assets, fair value disclosure	$ 163,908	$ 170,097

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details)	Sep. 30, 2021USD ($)
Fair Value Disclosures [Abstract]
Fair value, assets, level 1 to level 2 transfers, amount	$ 0
Fair value, assets, level 2 to level 1 transfers, amount	0
Fair value, liabilities, level 1 to level 2 transfers, amount	0
Fair value, liabilities, level 2 to level 1 transfers, amount	$ 0

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Total prepaid expenses and other current assets	$ 1,913	$ 4,582

Property and Equipment, Net - S

Property and Equipment, Net - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Property Plant And Equipment Net [Abstract]
Machinery and equipment	$ 6,660	$ 6,139
Leasehold improvements	579	579
Furniture and fixtures	318	459
Total property and equipment, gross	7,557	7,177
Less: Accumulated depreciation and amortization	(4,238)	(3,532)
Total property and equipment, net	$ 3,319	$ 3,645

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Feb. 29, 2020
Property, Plant and Equipment [Line Items]
Depreciation and amortization expense	$ 300	$ 300	$ 932	$ 1,011
Leasehold improvements	579		579		$ 579
Accumulated depreciation related to those leasehold improvements	4,238		4,238		3,532
Property and equipment, net	$ 3,319		$ 3,319		$ 3,645
Loss on termination of operating lease				108
2016 Lease [Member]
Property, Plant and Equipment [Line Items]
Leasehold improvements						$ 2,700
Accumulated depreciation related to those leasehold improvements						$ 1,300
Loss on termination of operating lease				100
2016 Lease [Member] | Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, net		$ 1,400		$ 1,400

Accrued Expenses - Summary of A

Accrued Expenses - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Payables And Accruals [Abstract]
Accrued external research, development and manufacturing costs	$ 1,661	$ 3,085
Accrued employee compensation and benefits	2,512	2,682
Accrued licensing fees (Note 9)	786	743
Other	702	848
Total accrued expenses	$ 5,661	$ 7,358

Preferred Stock - Additional In

Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended					9 Months Ended
	Nov. 30, 2020	Jun. 30, 2020	May 31, 2020	Feb. 29, 2020	Jan. 31, 2020	Sep. 30, 2020
Temporary Equity [Line Items]
Proceeds from issuance of convertible preferred stock, net of issuance costs paid in the period						$ 42,248
Temporary equity, shares outstanding						16,904,219
Common stock shares issued upon conversion	17,800,084
Series D Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Issuance of Series D convertible preferred stock, net of issuance costs (Shares)		1,940,945	1,940,945	1,094,247	1,094,247
Share price		$ 13.9365	$ 13.9365	$ 13.9365	$ 13.9365
Proceeds from issuance of convertible preferred stock, net of issuance costs paid in the period		$ 27,000	$ 27,000	$ 15,200	$ 15,200

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ in Thousands	Oct. 20, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Stockbased Compensation [Line Items]
Expected annual dividend yield				0.00%
Stock based compensation expense due to modification of options		$ 2,781	$ 796	$ 6,193	$ 2,247
Unrecognized compensation expense related to unvested stock based awards		$ 21,300		$ 21,300
Unrecognized compensation expense expected period for recognition				2 years 8 months 12 days
2020 Incentive Award Plan [Member]
Stockbased Compensation [Line Items]
Common stock shares reserved for future issuance	2,690,415	2,552,357		2,552,357		2,079,230
2020 Incentive Award Plan [Member] | Maximum [Member]
Stockbased Compensation [Line Items]
Percentage of aggregate number of shares of common stock outstanding	5.00%
2020 Employee Stock Purchase Plan [Member]
Stockbased Compensation [Line Items]
Common stock shares reserved for future issuance	275,886	523,749		523,749
Shares issued				0
2020 Employee Stock Purchase Plan [Member] | Maximum [Member]
Stockbased Compensation [Line Items]
Common stock shares reserved for future issuance	3,724,461
Percentage of aggregate number of shares of common stock outstanding	1.00%
Research and Development Expense [Member]
Stockbased Compensation [Line Items]
Stock based compensation expense due to modification of options		$ 1,423	$ 296	$ 2,670	$ 811
Research and Development Expense [Member] | Stock Options [Member]
Stockbased Compensation [Line Items]
Stock based compensation expense due to modification of options				$ 900

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021USD ($)$ / sharesshares	Dec. 31, 2020USD ($)$ / sharesshares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares - Outstanding at December 31, 2020 | shares	4,039,894
Granted | shares	1,218,982
Exercised | shares	(278,445)
Forfeited or canceled | shares	(452,789)
Number of Shares - Outstanding at September 30, 2021 | shares	4,527,642	4,039,894
Number of Shares - Vested and expected to vest at September 30, 2021 | shares	4,527,642
Number of Shares - Options exercisable at September 30, 2021 | shares	1,742,018
Weighted Average Exercise Price - Outstanding at December 31, 2020 | $ / shares	$ 7.69
Granted | $ / shares	15.79
Exercised | $ / shares	4.06
Forfeited or canceled | $ / shares	10.38
Weighted Average Exercise Price - Outstanding at September 30, 2021 | $ / shares	9.83	$ 7.69
Weighted Average Exercise Price - Vested and expected to vest at September 30, 2021 | $ / shares	9.83
Weighted Average Exercise Price - Options exercisable at September 30, 2021 | $ / shares	$ 5.13
Weighted-Average Remaining Contractual Term	8 years 1 month 28 days	8 years 4 months 28 days
Weighted-Average Remaining Contractual Term - Vested and expected to vest	8 years 1 month 28 days
Weighted-Average Remaining Contractual Term - Options exercisable	6 years 11 months 26 days
Aggregate Intrinsic Value - Outstanding | $	$ 24,441	$ 85,993
Aggregate Intrinsic Value - Vested and expected to vest | $	24,441
Aggregate Intrinsic Value - Options exercisable | $	$ 16,223

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 2,781	$ 796	$ 6,193	$ 2,247
Research and Development Expense [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	1,423	296	2,670	811
General and Administrative Expense [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 1,358	$ 500	$ 3,523	$ 1,436

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Income tax benefits	$ 0	$ 0	$ 0	$ 0

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Other Commitments [Line Items]
Commitments and contingencies (Note 9)
Research and development	20,520,000	$ 13,910,000	52,942,000	$ 37,815,000
Discretionary contribution by the employer to defined contribution benefit plan	100,000	$ 100,000	300,000	300,000
Manufacturing Services Agreements [Member]
Other Commitments [Line Items]
Non-cancelable payments	0		0
Sublicense Agreement [Member]
Other Commitments [Line Items]
Research and development			0	$ 0
Massachusetts Institute Of Technology
Other Commitments [Line Items]
Commitments and contingencies (Note 9)	$ 800,000		$ 800,000		$ 700,000

Leases - Additional Information

Leases - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended			9 Months Ended
	Sep. 30, 2021	Feb. 29, 2020	Dec. 31, 2018	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Sep. 30, 2019
Lessee, Lease, Description [Line Items]
Leasehold improvements	$ 579			$ 579		$ 579
Loss on termination of operating lease					$ 108
Operating lease right-of-use assets	72,282			72,282		48,360
Operating lease liability				31,306	17,049
Office Building [Member] | Watertown Massachusetts [Member] | Two Thousand And Eighteen Lease Agreement [Member]
Lessee, Lease, Description [Line Items]
Lessee operating lease month of expiry			2029-11
Lessee operating lease renewal term			5 years
Initial annual base rent			$ 3,800
Initial annual base rent escalation percentage			3.00%
Letter of credit outstanding			$ 2,300
Leasehold improvements			$ 9,800
Office Building [Member] | Watertown Massachusetts [Member] | Two Thousand And Sixteen Lease Agreement
Lessee, Lease, Description [Line Items]
Increase (decrease) in operating lease right of use assets		$ 2,100
Increase (decrease) in leasehold improvements operating lease		1,400
Increase (decrease) in operating lease liability		$ 3,400
Loss on termination of operating lease					$ 100
Machinery and Equipment [Member] | Embedded Lease [Member]
Lessee, Lease, Description [Line Items]
Operating lease term							2 years
Operating lease right-of-use assets	72,300			72,300		$ 48,400
Machinery and Equipment [Member] | Embedded Lease [Member] | Embedded Lease Amendment Agreement One [Member]
Lessee, Lease, Description [Line Items]
Lessee operating lease month of expiry					2022-08
Lessee operating lease renewal term					2 years
Machinery and Equipment [Member] | Embedded Lease [Member] | Embedded Lease Amendment Agreement Two [Member]
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets	31,300			$ 31,300
Operating lease liability	$ 31,300

Leases - Summary of Components

Leases - Summary of Components of Lease Cost and Other Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Lease cost:
Operating lease cost	$ 3,430	$ 3,231	$ 9,965	$ 9,345
Variable lease cost	425	306	1,428	908
Short-term lease cost				21
Lease Cost	$ 3,855	$ 3,537	$ 11,393	$ 10,274
Other information:
Weighted-average remaining lease term (in years)	6 years 4 months 24 days		6 years 4 months 24 days		6 years 8 months 12 days
Weighted-average discount rate	7.60%		7.60%		7.20%

Leases - Summary of Supplementa

Leases - Summary of Supplementary Cash Flow Information Relating to Operating Leases (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Leases [Abstract]
Operating cash flows from operating leases	$ 9,424	$ 8,820

License and Collaboration Agr_3

License and Collaboration Agreements - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended					9 Months Ended
	Oct. 31, 2018	Sep. 30, 2021	Sep. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
License And Collaboration Agreements [Line Items]
Contract with customer liability revenue recognized		$ 4,755,000	$ 6,121,000				$ 14,748,000	$ 18,511,000
Contract with customer liability		30,553,000	47,688,000		$ 40,453,000		$ 30,553,000	47,688,000	$ 35,308,000	$ 45,201,000	$ 51,918,000
2017 License and Collaboration Agreement With Roche [Member] | Roche [Member]
License And Collaboration Agreements [Line Items]
Term of license and collaboration agreement							2 years
Renewal term of license and collaboration agreement							1 year
Upfront payment received towards technology access fee							$ 5,000,000
Maximum reimbursement receivable for research and development costs incurred		1,000,000					1,000,000
Milestone payment receivable		7,000,000					7,000,000
Estimated costs to be incurred to satisfy performance obligation		100,000	0				400,000	0
Contract with customer liability revenue recognized		300,000	100,000				1,000,000	400,000
Contract with customer liability current		200,000					200,000
2017 License and Collaboration Agreement With Roche [Member] | Roche [Member] | Maximum [Member]
License And Collaboration Agreements [Line Items]
Annual maintenance fee receivable later than Five years		900,000					900,000
2017 License and Collaboration Agreement With Roche [Member] | Roche [Member] | Minimum [Member]
License And Collaboration Agreements [Line Items]
Annual maintenance fee receivable later than Five years		500,000					500,000
2017 License and Collaboration Agreement With Roche [Member] | Roche [Member] | Tranche One [Member]
License And Collaboration Agreements [Line Items]
Reimbursement receivable for research and development costs incurred		500,000					500,000
2017 License and Collaboration Agreement With Roche [Member] | Roche [Member] | Tranche Two [Member]
License And Collaboration Agreements [Line Items]
Reimbursement receivable for research and development costs incurred		500,000					500,000
2018 License and Collaboration Agreement With Roche [Member]
License And Collaboration Agreements [Line Items]
Milestone payment receivable on exercise of option rights		100,000,000					100,000,000
2018 License and Collaboration Agreement With Roche [Member] | Roche [Member]
License And Collaboration Agreements [Line Items]
Upfront payment received towards technology access fee	$ 45,000,000
Estimated costs to be incurred to satisfy performance obligation		400,000	0				400,000	0
Contract with customer liability current		21,200,000					21,200,000			24,700,000
Milestone payment receivable based on product		1,600,000					1,600,000
Performance obligation revenue recognized		4,500,000	$ 6,000,000				13,600,000	$ 18,100,000
Contract with customer liability		30,400,000					30,400,000			$ 44,000,000
2018 License and Collaboration Agreement With Roche [Member] | Roche [Member] | Development Milestone [Member]
License And Collaboration Agreements [Line Items]
Milestone payment receivable based on product		217,000,000					217,000,000
Performance obligation transaction price				$ 20,000,000		$ 10,000,000
2018 License and Collaboration Agreement With Roche [Member] | Roche [Member] | Development Milestone [Member] | First Patient Doosing Phase One Clinical Trial [Member]
License And Collaboration Agreements [Line Items]
Milestone payment received				$ 20,000,000
2018 License and Collaboration Agreement With Roche [Member] | Roche [Member] | Regulatory Milestone [Member]
License And Collaboration Agreements [Line Items]
Milestone payment receivable based on product		240,000,000					240,000,000
2018 License and Collaboration Agreement With Roche [Member] | Roche [Member] | Regulatory Milestone [Member] | Preclinical Data Submitted To FDA For Approval [Member]
License And Collaboration Agreements [Line Items]
Milestone payment received						$ 10,000,000
2018 License and Collaboration Agreement With Roche [Member] | Roche [Member] | Sales Milestone [Member]
License And Collaboration Agreements [Line Items]
Milestone payment receivable based on product		1,200,000,000					1,200,000,000
2018 License and Collaboration Agreement With Roche [Member] | Roche [Member] | APC [Member]
License And Collaboration Agreements [Line Items]
Amount payable on exercise of option rights to use the license		15,000,000					15,000,000
2018 License and Collaboration Agreement With Roche [Member] | Roche [Member] | TCL [Member]
License And Collaboration Agreements [Line Items]
Contract liabilities current reclassified to non current					$ 9,200,000
2018 License and Collaboration Agreement With Roche [Member] | Roche [Member] | Maximum [Member] | TCL [Member]
License And Collaboration Agreements [Line Items]
Amount payable on exercise of option rights to use the license		100,000,000					100,000,000
2018 License and Collaboration Agreement With Roche [Member] | Roche [Member] | Minimum [Member] | TCL [Member]
License And Collaboration Agreements [Line Items]
Amount payable on exercise of option rights to use the license		$ 50,000,000					$ 50,000,000

License and Collaboration Agr_4

License and Collaboration Agreements - Additional Information (Detail1) - Roche [Member] - Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2021-10-01	Sep. 30, 2021
2017 License and Collaboration Agreement With Roche [Member]
License And Collaboration Agreements [Line Items]
Remaining period over which the performance obligation is to be satisfied	9 months
2018 License and Collaboration Agreement With Roche [Member] | Minimum [Member]
License And Collaboration Agreements [Line Items]
Remaining period over which the performance obligation is to be satisfied	3 months
2018 License and Collaboration Agreement With Roche [Member] | Maximum [Member]
License And Collaboration Agreements [Line Items]
Remaining period over which the performance obligation is to be satisfied	9 months

License and Collaboration Agr_5

License and Collaboration Agreements - Summary of Changes in the Total Contract Liability (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Research And Development [Abstract]
Balance at beginning of period	$ 35,308	$ 51,918	$ 45,201	$ 40,453
Deferral of revenue		1,891		25,746
Other			100
Recognition of deferred revenue	(4,755)	(6,121)	(14,748)	(18,511)
Balance at end of period	$ 30,553	$ 47,688	$ 30,553	$ 47,688

Net Loss Per Share - Summary of

Net Loss Per Share - Summary of basic and diluted net loss per share attributable to common stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net loss	$ (22,450)	$ (12,351)	$ (56,919)	$ (32,920)
Weighted-average common shares outstanding, basic and diluted	28,050,130	1,758,039	27,421,839	1,744,948
Net loss per share attributable to common stockholders, basic and diluted	$ (0.80)	$ (7.03)	$ (2.08)	$ (18.87)

Net Loss Per Share - Summary _2

Net Loss Per Share - Summary of potentially dilutive shares excluded from the calculation of diluted net loss (Detail) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount	4,527,642	21,546,573	4,527,642	21,546,573
Redeemable Convertible Preferred Stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount		17,800,084		17,800,084
Warrant [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount		2,038		2,038
Share-based Payment Arrangement, Option [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount	4,527,642	3,744,451	4,527,642	3,744,451

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) $ in Millions	1 Months Ended
	Oct. 31, 2021USD ($)
Subsequent Event [Member] | 2018 Roche Agreement [Member]
Subsequent Event [Line Items]
Milestone payment receivable on combination	$ 3