The authors describe the discovery and characterization of an X chromosome-linked mouse mutant (mdx) that exhibits symptoms and histological lesions characteristic of muscular dystrophy. The mdx mutation arises spontaneously in the C57BL/10 colony and results in elevated plasma levels of muscle creatine kinase and pyruvate kinase. The mutants show mild clinical symptoms but are viable and fertile. Linkage analysis with four X chromosome loci indicates that the mdx gene maps to the Hq Bpa region, with a gene order of mdx-Tfm-Pgk-1-Ags, similar to the human X chromosome. The mdx mutant shares several histopathological features with human Duchenne and Becker muscular dystrophy, including elevated muscle enzyme levels and progressive muscle degeneration. The authors conclude that the mdx mutant is a promising animal model for X chromosome-linked muscular dystrophy.The authors describe the discovery and characterization of an X chromosome-linked mouse mutant (mdx) that exhibits symptoms and histological lesions characteristic of muscular dystrophy. The mdx mutation arises spontaneously in the C57BL/10 colony and results in elevated plasma levels of muscle creatine kinase and pyruvate kinase. The mutants show mild clinical symptoms but are viable and fertile. Linkage analysis with four X chromosome loci indicates that the mdx gene maps to the Hq Bpa region, with a gene order of mdx-Tfm-Pgk-1-Ags, similar to the human X chromosome. The mdx mutant shares several histopathological features with human Duchenne and Becker muscular dystrophy, including elevated muscle enzyme levels and progressive muscle degeneration. The authors conclude that the mdx mutant is a promising animal model for X chromosome-linked muscular dystrophy.