Dimension Therapeutics, Inc. (DMTX) develops adeno-associated virus (AAV)-based gene therapies targeting rare genetic diseases associated with the liver, including hemophilia A and B; ornithine transcarbamylase (OTC) deficiency; glycogen storage disease type Ia (GSDIa); phenylketonuria (PKU); Wilson disease; and citrullinemia type I. The company's lead product candidates comprise DTX101, an AAVrh10 vector delivering Factor IX (FIX) gene expression for hemophilia B; DTX201, an FVIII gene therapy program for hemophilia A developed in collaboration with Bayer; DTX301 (avalotcagene ontaparvovec), an AAV therapy for late-onset OTC deficiency currently in a Phase 3 Enh3ance study; and DTX401, an AAV8 gene therapy for GSDIa with a rolling Biologics License Application (BLA) submission to the FDA initiated in August 2025 and positive long-term Phase 3 data showing sustained reductions in daily cornstarch intake. Founded in 2013 and headquartered in Cambridge, Massachusetts, Dimension Therapeutics operates as a wholly-owned subsidiary of Ultragenyx Pharmaceutical Inc. following its $151 million cash acquisition in 2017, which succeeded an earlier terminated merger agreement with REGENXBIO. Recent developments include Ultragenyx's advancement of DTX401 toward full BLA completion in Q4 2025 with RMAT, Fast Track, and Orphan Drug designations; ongoing Phase 3 enrollment for DTX301; discontinuation of DTX101 after Phase 1/2 trials failed to achieve targeted FIX levels despite acceptable safety; and maintenance of Dimension's gene therapy R&D and manufacturing capabilities in Massachusetts to support pipeline progression.