Muscular dystrophy is a group of diseases that cause progressive weakness and loss of muscle mass. In muscular dystrophy, abnormal genes (mutations) interfere with the production of proteins needed to form healthy muscle. There are many different kinds of muscular dystrophy. Symptoms of the most common variety begin in childhood, primarily in boys. Other types don't surface until adulthood. Some people who have muscular dystrophy will eventually lose the ability to walk. Some may have trouble breathing or swallowing.