Duchenne muscular dystrophy is a genetic disease that causes muscle weakness and wasting.Children born with the disease have a fault, known as a mutation, on their dystrophin gene. The fault on the gene means that they cannot produce dystrophin, which is a protein that protects muscles. Without dystrophin, muscles get damaged more easily and so muscle strength and function is weakened.
Duchenne muscular dystrophy almost always affect boys, and they tend to be diagnosed before the age of 5.
more information is available here