X0306 4 2021-08-25 0 0001690585 Magenta Therapeutics, Inc. MGTA 0001688327 Third Rock Ventures IV, L.P. C/O THIRD ROCK VENTURES, LLC 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001688326 Third Rock Ventures GP IV, L.P. 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001688325 TRV GP IV, LLC 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 Common Stock 2021-08-25 4 J 0 1250000 D 6758204 D On August 25, 2021, Third Rock Ventures IV, L.P. ("TRV IV") distributed for no consideration, 1,250,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to Third Rock Ventures GP IV, L.P. ("TRV GP IV"), the general partner of TRV IV, representing each such partner's pro rata interest in such Shares. On the same date, TRV GP IV distributed, for no consideration, the Shares it received in the distribution by TRV IV to its partners, representing each such partner's pro rata interest in such Shares. All of the aforementioned distributions were made in accordance with the exemptions afforded by Rules 16a-13 and 16a-9 of the Securities Exchange Act of 1934, as amended. These shares are directly held by TRV IV. The general partner of TRV IV is TRV GP IV. The general partner of TRV GP IV is TRV GP IV, LLC ("TRV GP IV LLC"). Each of TRV GP IV and TRV GP IV LLC disclaims beneficial ownership of the shares except to the extent of its pecuniary interest therein, if any, and this report shall not be deemed an admission that it is the beneficial owner of such shares. /s/ Kevin Gillis, Chief Operating Officer of TRV GP IV, LLC, general partner of Third Rock Ventures GP IV, L.P., general partner of Third Rock Ventures IV, L.P. 2021-08-26 /s/ Kevin Gillis, Chief Operating Officer of TRV GP IV, LLC, general partner of Third Rock Ventures GP IV, L.P. 2021-08-26 /s/ Kevin Gillis, Chief Operating Officer of TRV GP IV, LLC 2021-08-26