Duchenne muscular dystrophy (DMD) is a genetic disease. The main sign of DMD is muscle weakness that worsens over time. Before age five, the muscles in the legs, arms, and trunk begin to weaken. Later in the disease, the heart and respiratory muscles weaken.

DMD is caused by a genetic mutation. The mutation causes the gene to make inadequate amounts of a protein called dystrophin. This protein is needed to keep muscles intact.