X0206 3 2015-10-21 0 0001592288 Dimension Therapeutics, Inc. DMTX 0001601264 Dybbs Michael C/O NEW LEAF VENTURE PARTNERS 7 TIMES SQUARE, SUITE 3502 NEW YORK NY 10036 1 0 0 0 Series B Preferred Stock Common Stock 1097745 I See Footnote These securities are shares of Series B Preferred Stock (the "Series B Shares") of the Issuer, and are convertible at any time into shares of the Issuer's Common Stock at the holders' election and automatically upon the closing of the Issuer's initial public offering, into the number of shares of Common Stock shown in column 3 above. These securities are held by New Leaf Ventures III, L.P. ("NLV-III") and New Leaf Growth Fund I, L.P. ("NLGF"). New Leaf Venture Associates III, L.P. ("NLVA-III") is the general partner of NLV-III and New Leaf Venture Management III, L.L.C. ("NLVM-III") is the general partner of NLVA-III. New Leaf Growth Associates I, L.P. ("NLGA") is the general partner of NLGF and NLVM-III is the general partner of NLGA. Reporting Person is employed by New Leaf Venture Partners, L.L.C., which is affiliated with the above entities and disclaims beneficial ownership of the shares held by NLV-III and NLGF except to the extent of his pecuniary interests therein if any. Exhibit 24.1: Limited Power of Attorney /s/ Mary Thistle, as Attorney-in-Fact for Michael Dybbs 2015-10-21