Muscular dystrophy is a group of genetic conditions characterized by progressive muscle weakness and wasting. The Duchenne and Becker types of muscular dystrophy primarily affect the skeletal muscles, which are used for movement, and the muscles of the heart. Duchenne muscular dystrophy is the most common form of muscular dystrophy in children. Duchenne and Becker muscular dystrophies have similar signs and symptoms and are caused by mutations in the same gene. The two conditions differ in their severity and age of onset. In people with Duchenne muscular dystrophy, muscle weakness tends to appear early in childhood and progress rapidly. Heart problems, which can become life-threatening, typically begin in the teenage years. The signs and symptoms of Becker muscular dystrophy are usually milder and exhibit a large range of variation. In general, they become apparent later in childhood or adolescence and progress at a much slower rate.