X0306 4 2017-11-07 1 0001592288 Dimension Therapeutics, Inc. DMTX 0001653056 Wadsworth Samuel C. C/O DIMENSION THERAPEUTICS, INC. 840 MEMORIAL DRIVE, 4TH FLOOR CAMBRIDGE MA 02139 0 1 0 0 Chief Scientific Officer Common Stock 2017-11-07 4 D 0 278035 D 0 D Stock Option (Right to Buy) 7.14 2017-11-07 4 D 0 50000 D 2026-01-31 Common Stock 50000 0 D Stock Option (Right to Buy) 1.90 2017-11-07 4 D 0 60000 D 2027-01-31 Common Stock 60000 0 D Disposed of pursuant to the Agreement and Plan of Merger (the "Merger Agreement"), dated October 2, 2017, between the Issuer, Ultragenyx Pharmaceutical Inc. ("Ultragenyx"), and Mystic River Merger Sub Inc., a direct, wholly-owned subsidiary of Ultragenyx, in exchange for a cash payment of $6.00 per share. This stock option, which vests in equal monthly installments over 48 months beginning January 27, 2016, was assumed by Ultragenyx and converted in accordance with the exchange ratio as set forth in the Merger Agreement. This stock option, which vests in equal monthly installments over 48 months beginning January 18, 2017, was assumed by Ultragenyx and converted in accordance with the exchange ratio as set forth in the Merger Agreement. /s/ Mary Thistle, as Attorney-in-Fact for Samuel C. Wadsworth 2017-11-07