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A group of genetic diseases that are characterized by progressive weakness and degeneration of the skeletal muscles that control movement. There are many forms of muscular dystrophy, some noticeable at birth (congenital muscular dystrophy), others in adolescence (Becker MD), but the three most common types are Duchenne, facioscapulohumeral, and myotonic which differ in terms of pattern of inheritance, age of onset, rate of progression, and distribution of weakness. Duchenne MD primarily affects boys and is the result of mutations in the gene that regulates dystrophin, a protein involved in maintaining the integrity of muscle fiber. Onset is between three to five years and progresses rapidly. Most boys become unable to walk at age 12, and by age 20 have to use a respirator to breathe. Facioscapulohumeral MD appears in adolescence and causes progressive weakness in facial muscles and certain muscles in the arms and legs. It progresses slowly and can vary in symptoms from mild to disabling. Myotonic MD varies in the age of onset and is characterized by myotonia (prolonged muscle spasm) in the fingers and facial muscles; a floppy-footed, high-stepping gait; cataracts; cardiac abnormalities; and endocrine disturbances. Individuals with myotonic MD have long faces and drooping eyelids; men have frontal baldness.