How does a faulty gene cause DMD?

The body is made of cells, and each cell has a 'control centre' which contains genes. The genes are made of DNA. The genes control how the cells make chemicals (proteins). Every cell contains many different genes and makes many different proteins. One of these proteins is the dystrophin protein which is involved in DMD.

In DMD there is a fault in the the gene which makes dystrophin. So the muscle cells cannot make dystrophin (or make only a little of it). This leads to muscle damage, as explained above.