Through a process that has come to be known as reverse genetics, the gene and gene product involved in Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) have been identified. The DMD/BMD gene is over 2 million base pairs in size and over 50% of DMD/BMD patients harbor submicroscopic deletions for portions of the gene. The gene product, named dystrophin, is 400 Kd in size. Dystrophin is present in skeletal, cardiac, and smooth muscles, as well as brain. The protein is absent or altered in DMD/BMD patient muscle. The normal function of dystrophin and the reasons why its alteration results in the DMD/BMD phenotypes are presently unknown. The discoveries to date, however, provide a starting point for investigating the fundamental pathogenetic mechanisms involved in DMD/BMD.