Muscular dystrophies (dystrophinopathies) are progressive hereditary degenerative diseases of skeletal muscles due to an absence or deficiency of the protein dystrophin. Dystrophin and the associated proteins form a complex system that connects the intracellular cytoskeleton to the extracellular matrix. The normalcy of this system is critical for maintaining the integrity of the delicate, elastic muscle membrane (sarcolemma) and the muscle fiber. The responsible gene is located on the short arm of the X chromosome.

The most common mechanisms of mutation are deletions and duplications of base pairs largely clustered in the "hot spot," a DNA sequence associated with an abnormally high frequency of mutation or recombination. The type of deletion determines whether dystrophin is absent from the muscle or present in a reduced, altered form. This has an important clinical significance because the former is usually associated with the severe Duchenne's variety of the disease (DMD...