Duchenne muscular dystrophy is an X-linked developmental disorder caused by mutations in the dystrophin gene leading to progressive muscular weakness and premature death due to respiratory and/or cardiac failure. RGX-202 is a recombinant adeno-associated virus of serotype 8 (AAV8) with an optimized human microdystrophin transgene and a promoter designed to increase expression in muscle (Spc5-12). The RGX-202 microdystrophin transgene is designed to encode key elements of full-length dystrophin including the extended coding region of the C-terminal (CT) domain. The CT domain is known to recruit ?-dystrobrevin and ?-syntrophin to the dystrophin-associated protein complex (DAPC) that contributes to maintaining membrane integrity and cellular signaling during muscle contraction and relaxation. RGX-202 was administered intravenously to young male mdx mice and evaluated over 12 weeks. RGX-202-administered mdx mice showed a dose-proportional increase of RGX-202 vector DNA and microdystrophin protein in the skeletal and cardiac muscles. RGX-202-administered mdx mice exhibited significant improvements in muscle strength and gait, respectively, measured by in vitro force (specific force and eccentric contractions) and fine motor kinematic analysis. Additionally, a dose-dependent reduction of hyperintense lesions associated with muscle edema was observed using T2 magnetic resonance imaging. Consistent with these results, the dystrophic pathology (i.e., inflammation, degeneration, and regeneration) was improved in the RGX-202-administered mdx mice. Immunofluorescence data revealed that increased RGX-202 microdystrophin expression was correctly localized to the sarcolemma of the skeletal muscles and DAPC expression also was restored. Our data demonstrates that a single intravenous injection (IV) dose administration of RGX-202 in mdx mice has provided notable improvements in muscle function and dystrophic muscle pathology, and increased RGX-202 vector DNA and microdystrophin protein expression. We are currently developing RGX-202 as a candidate for microdystrophin gene therapy for DMD.