Muscular dystrophies primarily affect skeletal muscle and are characterized by progressive muscle wasting and weakness. Although these diseases have been clinically recognized for some time, genetic defects in a number of muscular dystrophies only recently have been identified. One of the most important advances in understanding the molecular genetics of neuromuscular diseases has been the cloning of the gene encoding dystrophin, the protein that is absent in the muscle of patients with Duchenne and Becker muscular dystrophy. Several dystrophin-associated proteins have been identified. Components of the dystrophin-glycoprotein complex are being characterized, and evidence indicates that proteins of this complex may be responsible for other forms of muscular dystrophy.