Related Parties	12 Months Ended
Dec. 31, 2019
Related Parties
Related Parties	15. Related Parties In April 2013, the Company entered into a services agreement with Flagship Ventures Management, Inc. (“Flagship”), an affiliate of one of its principal stockholders, to provide general and administrative services to the Company, including certain consulting services and the provision of employee health and dental benefit plans for the Company’s employees. The Company recorded general and administrative expense and made cash payments for services received under this agreement of $1.3 million and $0.9 million during the years ended December 31, 2018 and 2017, respectively. The Company did not receive services and did not pay any amounts under this agreement during the year ended December 31, 2019. As of December 31, 2019 and 2018, the Company had no amounts payable to Flagship for costs related to the services agreement. In January 2017, the Company loaned $0.7 million to the chairman of its board of directors to purchase shares of common stock pursuant to a promissory note and a restricted stock agreement (see Note 10). In May 2017, the Company loaned $1.8 million to the chairman of its board of directors to purchase shares of common stock pursuant to a promissory note and a restricted stock agreement (see Note 10). The January 2017 promissory note provided that the unpaid principal amount of the loan bore interest at 1.97% annually, and the May 2017 promissory note provided that the unpaid principal amount of the loan bore interest at 2.04% annually. Interest was payable annually or was converted to principal and payable at the maturity date. The maturity date of the promissory notes occurred on the earliest of (i) seven years from the issuance date of the notes, (ii) 60 days following the date of termination of services of the borrower, and (iii) immediately prior to an initial filing of a registration statement by the Company. The promissory notes were partial‑recourse and secured by a pledge of the shares of common stock purchased with the promissory notes. As of December 31, 2017, no amounts were due to the Company and no amounts had been received by the Company as repayment of these promissory notes. On June 21, 2018, the aggregate principal balance of both promissory notes of $2.5 million and all interest that had accrued thereon, totaling $0.1 million, was forgiven by the Company and the promissory notes were terminated. In April 2017, the Company loaned $0.2 million to an executive officer of the Company to purchase shares of common stock pursuant to two promissory notes and two restricted stock agreements (see Note 10). The promissory notes provided that the unpaid principal amount of the loans bore interest at 2.05% annually, and interest was payable annually or was converted to principal and payable at the maturity date. The maturity date of the promissory notes occurred on the earliest of (i) seven years from the issuance date of the notes, (ii) 60 days following the date of termination of employment of the borrower, and (iii) immediately prior to an initial filing of a registration statement by the Company. The promissory notes were partial‑recourse and secured by a pledge of the shares of common stock purchased with the promissory notes. As of December 31, 2017, no amounts were due to the Company and no amounts had been received by the Company as repayment of these promissory notes. On June 21, 2018, the aggregate principal balance of both promissory notes of $0.2 million and all interest that had accrued thereon, totaling less than $0.1 million, was repaid in full by the executive officer and the promissory notes were terminated. In February 2019, the Company entered into a sublease agreement with an affiliate of one of its principal stockholders to sublease all office and laboratory space under one of the Company’s operating leases in Cambridge, Massachusetts. The term of the sublease agreement commenced in February 2019 and expires at the end of the Company’s lease agreement with the landlord in September 2021, with no option to extend (see Note 13). Under this agreement, the Company recorded other income of $0.9 million during the year ended December 31, 2019. The Company received cash payments of $0.5 million during the year ended December 31, 2019. As of December 31, 2019, the Company recorded accounts receivable of $0.1 million under this agreement.

15. Related Parties

In April 2013, the Company entered into a services agreement with Flagship Ventures Management, Inc. (“Flagship”), an affiliate of one of its principal stockholders, to provide general and administrative services to the Company, including certain consulting services and the provision of employee health and dental benefit plans for the Company’s employees. The Company recorded general and administrative expense and made cash payments for services received under this agreement of $1.3 million and $0.9 million during the years ended December 31, 2018 and 2017, respectively. The Company did not receive services and did not pay any amounts under this agreement during the year ended December 31, 2019. As of December 31, 2019 and 2018, the Company had no amounts payable to Flagship for costs related to the services agreement.

In January 2017, the Company loaned $0.7 million to the chairman of its board of directors to purchase shares of common stock pursuant to a promissory note and a restricted stock agreement (see Note 10). In May 2017, the Company loaned $1.8 million to the chairman of its board of directors to purchase shares of common stock pursuant to a promissory note and a restricted stock agreement (see Note 10). The January 2017 promissory note provided that the unpaid principal amount of the loan bore interest at 1.97% annually, and the May 2017 promissory note provided that the unpaid principal amount of the loan bore interest at 2.04% annually. Interest was payable annually or was converted to principal and payable at the maturity date. The maturity date of the promissory notes occurred on the earliest of (i) seven years from the issuance date of the notes, (ii) 60 days following the date of termination of services of the borrower, and (iii) immediately prior to an initial filing of a registration statement by the Company. The promissory notes were partial‑recourse and secured by a pledge of the shares of common stock purchased with the promissory notes. As of December 31, 2017, no amounts were due to the Company and no amounts had been received by the Company as repayment of these promissory notes. On June 21, 2018, the aggregate principal balance of both promissory notes of $2.5 million and all interest that had accrued thereon, totaling $0.1 million, was forgiven by the Company and the promissory notes were terminated.

In April 2017, the Company loaned $0.2 million to an executive officer of the Company to purchase shares of common stock pursuant to two promissory notes and two restricted stock agreements (see Note 10). The promissory notes provided that the unpaid principal amount of the loans bore interest at 2.05% annually, and interest was payable annually or was converted to principal and payable at the maturity date. The maturity date of the promissory notes occurred on the earliest of (i) seven years from the issuance date of the notes, (ii) 60 days following the date of termination of employment of the borrower, and (iii) immediately prior to an initial filing of a registration statement by the Company. The promissory notes were partial‑recourse and secured by a pledge of the shares of common stock purchased with the promissory notes. As of December 31, 2017, no amounts were due to the Company and no amounts had been received by the Company as repayment of these promissory notes. On June 21, 2018, the aggregate principal balance of both promissory notes of $0.2 million and all interest that had accrued thereon, totaling less than $0.1 million, was repaid in full by the executive officer and the promissory notes were terminated.

In February 2019, the Company entered into a sublease agreement with an affiliate of one of its principal stockholders to sublease all office and laboratory space under one of the Company’s operating leases in Cambridge, Massachusetts. The term of the sublease agreement commenced in February 2019 and expires at the end of the Company’s lease agreement with the landlord in September 2021, with no option to extend (see Note 13). Under this agreement, the Company recorded other income of $0.9 million during the year ended December 31, 2019. The Company received cash payments of $0.5 million during the year ended December 31, 2019. As of December 31, 2019, the Company recorded accounts receivable of $0.1 million under this agreement.