The musculary dystrophies are a group of diseases characterized by progressive degeneration and/or loss of muscle fibers that result in loss of muscle strength. All or nearly all of them have a hereditary origin but details of the type of genetic defect and of the prognosis for the disease vary from type to type. Duchenne muscular dystrophy is the most common form, though there are at least 6 other forms of the disease. Several million people throughout the world have a form of muscular dystrophy. In the United States, it is estimated that 250,000 suffer from a form of muscular dystrophy.