Muscular dystrophy is the most common inherited muscle disorder and is restricted to males. It causes weakness in the limbs as muscles are gradually supplanted by useless connective tissue, leading to disability and then death -- usually before the sufferer passes his mid-20s.
In 1986, the defect that leads to Duchenne's muscular dystrophy was isolated to a bit of DNA that makes a protein called dystrophin. The lack of dystrophin causes muscles to waste away. However, its exact role is unclear. In healthy limbs, dystrophin is attached to the inside of the membrane which surrounds each muscle cell (myoblast) and somehow keeps the cell working. The disease seemed to be linked with an excess of calcium in degenerating muscle, which is associated with enzymes which destroy the muscle tissue. The key could be an imperfectly operating "calcium-leak channel" in the cell membrane, allowing excessive calcium to leak into cells with no dystrophin.
The disease afflicts about one in every 3,400 boys.