Duchenne and Becker muscular dystrophies

Duchenne muscular dystrophy (DMD) and the milder Becker
form (BMD) are X linked recessive disorders causing
progressive proximal muscle weakness, associated with elevation
of serum creatine kinase levels. Weakness of the diaphragm and
intercostal muscles leads to respiratory insufficiency, and
involvement of the myocardium causing dilated
cardiomyopathy is common.
Both DMD and BMD result from mutations in the gene
encoding dystrophin, located at Xp21. The gene is one of the
largest identified covering approximately 2.5 megabases of
DNA and having 79 exons. Two-thirds of cases are caused by
deletion of one or more of the dystrophin exons that cluster in
two hot-spots within the gene. Large duplications account for a
further 5–10% of cases. The remainder of cases are due to a
variety of point mutations.