X0306 4 2016-05-02 0 0001592288 Dimension Therapeutics, Inc. DMTX 0000315066 FMR LLC 245 SUMMER STREET BOSTON MA 02210 0 0 1 1 See Remark 1 Common Stock 2016-05-02 4 J 0 2106784 0 D 4893322 I See Footnote 1 F-Prime Capital Partners Healthcare Fund III LP made a pro-rata distribution to its partners, without payment of consideration, of 5,950,090 shares of Dimension Therapeutics Inc. previously reported by FMR LLC. F-Prime Capital Partners Healthcare Fund III LP distributed 3,838,249 shares to Impresa Fund III Limited Partnership, 44,862 shares to F-Prime Capital Partners Healthcare Advisors Fund III LP (FPCPHA) and 2,066,979 shares to a recipient whose shares are not subject to reporting by the undersigned (an Other Recipient). FPCPHA made a pro-rata distribution to its partners, without payment of consideration, of 5,057 shares to F-Prime Inc, a wholly-owned subsidiary of FMR LLC, and 39,805 shares to Other Recipients. Remark 1: Abigail P. Johnson is a Director, the Vice Chairman, the Chief Executive Officer and the President 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(f/k/a Beacon Bioventures Fund III Limited Partnership) is F-Prime Capital Partners Healthcare Advisors Fund III LP (f/k/a Beacon Bioventures Advisors Fund III Limited Partnership). F-Prime Capital Partners Healthcare Advisors Fund III LP is solely managed by Impresa Management LLC, its 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. 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 2016-05-03