Dystrophy refers to primary degenerative disease of the skeletal muscles. Duchenne's muscular dystrophy, a sex-linked disease, is the most important type. It is due to deletional mutation of a gene located on Xp21, which encodes for a muscle membrane protein, called dystrophin. In this picture, there is a wide variation of fiber size with some muscles undergoing hypertrophy and others undergo degeneration. With time, muscle fibers will be replaced by connective tissues, i.e. fat and fibrous tissues.

The prognosis is extremely poor : most boys died in their teens.