X0306 4 2016-02-08 0 0001650664 Editas Medicine, Inc. EDIT 0001569705 Third Rock Ventures III, L.P. C/O THIRD ROCK VENTURES, LLC, 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001637300 Third Rock Ventures GP III, L.P. 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001637299 TRV GP III, LLC 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001180428 LEVIN MARK J C/O THIRD ROCK VENTURES, LLC 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001192603 STARR KEVIN P C/O THIRD ROCK VENTURES, LLC 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001192595 TEPPER ROBERT I C/O THIRD ROCK VENTURES, LLC 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 Common Stock 2016-02-08 4 C 0 4656176 A 4656176 D Series A-1 Preferred Stock 2016-02-08 4 C 0 6628542 0.00 D Common Stock 2549437 0 D Series A-2 Preferred Stock 2016-02-08 4 C 0 5255300 0.00 D Common Stock 2021269 0 D Series B Preferred Stock 2016-02-08 4 C 0 222222 0.00 D Common Stock 85470 0 D Represents the total number of shares of Common Stock received by Third Rock Ventures III, L.P. ("TRV III") upon conversion of Series A-1, Series A-2 and Series B Preferred Stock in connection with the closing of the Issuer's initial public offering. The Series A-1, Series A-2 and Series B Preferred Stock converted into Common Stock on a 2.6-for-one basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A-1, Series A-2 and B Preferred Stock were convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Preferred Stock had no expiration date. The shares are directly held by TRV III. The general partner of TRV III is Third Rock Ventures GP III, L.P. ("TRV GP III"). The general partner of TRV GP III is TRV GP III, LLC ("TRV GP III LLC"). The individual managers of TRV GP III LLC are Mark Levin ("Levin"), Kevin Starr ("Starr") and Dr. Robert Tepper ("Tepper"). Each of TRV GP III, TRV GP III LLC, Levin, Starr and Tepper disclaims beneficial ownership of these shares except to the extent of its or his pecuniary interest therein, if any. Each of TRV GP III, TRV GP III LLC, Levin, Starr and Tepper disclaims beneficial ownership of these shares except to the extent of its or his pecuniary interest therein, if any. /s/ Kevin Gillis, Chief Financial Officer of TRV GP III, LLC, general partner of Third Rock Ventures GP III, L.P., general partner of Third Rock Ventures III, L.P. 2016-02-08 /s/ Kevin Gillis, Chief Financial Officer of TRV GP III, LLC, general partner of Third Rock Ventures GP III, L.P. 2016-02-08 /s/ Kevin Gillis, Chief Financial Officer of TRV GP III, LLC 2016-02-08 /s/ Kevin Gillis by power of attorney for Mark Levin 2016-02-08 /s/ Kevin Gillis by power of attorney for Kevin Starr 2016-02-08 /s/ Kevin Gillis by power of attorney for Dr. Robert Tepper 2016-02-08