Duchenne muscular dystrophy (DMD) is a genetically X-linked disorder of skeletal muscle, and is caused by primary problems of the dystrophin (Dys) gene. One in 3500 boys are affected with DMD, making DMD the most common of the human muscular dystrophies. Dystrophin has been localized to the muscle cell plasmalemma by electron-microscopic immunoperoxidase techniques, and appears to be a cytoskeletal protein. The main function of Dys is the maintenance of mechanical stability at the muscle cell membrane so normal stresses encountered during contraction can be withstood without membrane disruption.