Duchenne muscular dystrophy (DMD) is a severe muscle-wasting disorder affecting 1 in 3500 males. A less severe and less common form is Becker muscular dystrophy (BMD). Only recently has the basic defect in DMD and BMD been recognized: a region on the human X chromosome is disrupted by mutation. The primary transcript of the region was detected, and cDNA clones were isolated that encompassed the entire transcript. The sequence of the encoded protein was predicted from cDNA nucleotide sequence, portions of the protein were overexpressed in bacterial cells, and these peptides were used to generate immunoreagents against the DMD gene protein, dystrophin. The molecular genetic identification of this protein via analysis of mutations found in patients' material has led to a means of improved diagnosis of DMD/BMD in affected individuals and their family members. The severely affected DMD patients have little or no detectable dystrophin in their muscle, whereas BMD patients have nearly normal concentrations of an altered form of dystrophin; patients with all other neuromuscular diseases have normal dystrophin.