X0306 4 2017-11-07 1 0001592288 Dimension Therapeutics, Inc. DMTX 0001362520 Morris Arlene C/O DIMENSION THERAPEUTICS, INC. 840 MEMORIAL DRIVE, 4TH FLOOR CAMBRIDGE MA 02139 1 0 0 0 Stock Option (Right to Buy) 4.85 2017-11-07 4 D 0 44474 D 2025-08-11 Common Stock 44474 0 D Stock Option (Right to Buy) 7.08 2017-11-07 4 D 0 32069 D 2026-05-18 Common Stock 32069 0 D Stock Option (Right to Buy) 1.15 2017-11-07 4 D 0 17000 D 2027-05-24 Common Stock 17000 0 D This stock option, which vests in equal monthly installments over 48 months beginning June 3, 2015, was assumed by Ultragenyx Pharmaceutical Inc. ("Ultragenyx"), and converted in accordance with the exchange ratio as set forth in the Agreement and Plan of Merger (the "Merger Agreement"), dated October 2, 2017, between the Issuer, Ultragenyx and Mystic River Merger Sub Inc., a direct, wholly-owned subsidiary of Ultragenyx. These options were assumed by Ultragenyx and converted in accordance with the exchange ratio as set forth in the Merger Agreement. These options, which vest in full on the earlier of May 25, 2018 or the Issuer's next annual meeting of stockholders, subject to the director's continued service on the Board, were 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 Arlene Morris 2017-11-07