The mdx mouse is a murine genetic equivalent of the human X-linked lethal disorder, Duchenne muscular dystrophy (DMD). A number of studies utilizing the mdx mouse have demonstrated the feasibility of gene therapy for this disorder. Many such studies require the ability to determine rapidly the mdx genotype of experimental animals. Previous methods described to identify the mdx allele require multiple manipulations which are technically demanding. We now describe a simple and rapid method to detect the mdx and wild-type alleles in crude mouse DNA samples, by the mdx-amplification-resistant mutation system (ARMS) assay. With this system we correctly identified the mdx status of various transgene-containing animals in a rapid and simple fashion. We discuss the utility of this system for many other studies utilizing the mdx mouse as a model system.