X0306 4 2017-11-07 1 0001592288 Dimension Therapeutics, Inc. DMTX 0000315066 FMR LLC 245 SUMMER STREET BOSTON MA 02210 0 0 0 1 See Remark 1 Common Stock 2017-11-07 4 U 0 3838249 6 D 0 I Impresa Fund III Limited Partnership Common Stock 2017-11-07 4 U 0 1050016 6 D 0 I F-Prime Capital Partners Healthcare Fund III LP Common Stock 2017-11-07 4 U 0 5057 6 D 0 I F-Prime Inc. Remark 1: Abigail P. Johnson is a Director, the Chairman and the Chief Executive Officer of FMR LLC. Members of the Johnson family, including Abigail P. Johnson, are the predominant owners, directly or through trusts, of Series B voting common shares of FMR LLC, representing 49% of the voting power of FMR LLC. The Johnson family group and all other Series B shareholders have entered into a shareholders' voting agreement under which all Series B voting common shares will be voted in accordance with the majority vote of Series B voting common shares. Accordingly, through their ownership of voting common shares and the execution of the shareholders' voting agreement, members of the Johnson family may be deemed, under the Investment Company Act of 1940, to form a controlling group with respect to FMR LLC. The address of Abigail P. Johnson is c/o FMR LLC, 245 Summer Street, Boston, MA 02110. Remark 2: The general partner of F-Prime Capital Partners Healthcare Fund III LP is F-Prime Capital Partners Healthcare Advisors Fund III LP (FPCPHA). FPCPHA and Impresa Fund III Limited Partnership are each solely managed by Impresa Management LLC, their general partner and investment manager. Impresa Management LLC is owned, directly or indirectly, by various shareholders and employees of FMR LLC, including certain members of the Johnson family. F-Prime Inc. is a wholly-owned subsidiary of FMR LLC. Remark 3: The filing of this statement shall not be deemed to be an admission that, for purposes of Section 16 of the Securities Exchange Act of 1934 or otherwise, the undersigned are the beneficial owners of any securities reported herein. Marc R. Bryant, Duly authorized under Powers of Attorney, by and on behalf of FMR LLC and its direct and indirect subsidiaries, and Abigail P. Johnson 2017-11-08