X0306 4 2016-09-08 0 0001650664 Editas Medicine, Inc. EDIT 0001665594 Polaris Venture Management Co. VI, L.L.C. ONE MARINA PARK DRIVE, 10TH FLOOR BOSTON MA 02210 0 0 1 0 0001493463 Polaris Venture Partners VI, L.P. 1000 WINTER STREET, SUITE 3350 WALTHAM MA 02451 0 0 1 0 0001575691 Polaris Venture Partners Founders' Fund VI, L.P. 1000 WINTER STREET, SUITE 3350 WALTHAM MA 02451 0 0 1 0 Common Stock 2016-09-08 4 S 0 500000 16.80 D 4156173 I See Footnote 3,926,710 of these shares are owned directly by Polaris Venture Partners VI, L.P. and 229,463 of the shares are owned by Polaris Venture Partners Founders' Fund VI, L.P. (together with Polaris Venture Partners VI, L.P., the "Polaris Funds"). Polaris Venture Management Co. VI, L.L.C. ("Polaris Management") is the general partner of each of the Polaris Funds. Kevin Bitterman, a member of the Board of Directors of the Issuer, has an assignee interest in Polaris Management. Each of Polaris Management and Mr. Bitterman disclaim beneficial ownership of all the shares owned by the Polaris Funds and this report shall not be deemed an admission that either is the beneficial owner of such shares for purposes of Section 16 or for any other purpose, except to the extent of their pecuniary interests therein. /s/ Mary Blair, Attorney-in-Fact for Polaris Venture Management Co. VI, L.L.C. 2016-09-12 /s/ Mary Blair, Attorney-in-Fact for Polaris Venture Management Co. VI, L.L.C., the General Partner of Polaris Venture Partners VI, L.P. 2016-09-12 /s/ Mary Blair, Attorney-in-Fact for Polaris Venture Management Co. VI, L.L.C., the General Partner of Polaris Venture Partners Founders' Fund VI, L.P. 2016-09-12