Duchenne and Becker Muscular Dystrophy

The two forms of dystrophin-associated muscular dystrophies, known as Duchenne and Becker muscular dystrophy (DMD/BMD; OMIM 310200) are caused by genetic defects in the huge DMD gene (79 exons), located at Xp21 and coding for the 427-kDa protein known as dystrophin (1 ,2 ). DMD is the most frequent muscular disorder in boys, with an incidence of 1 in 3,500 males and a new mutation frequency of 1 in 10,000. Characteristics of the severe DMD phenotype are early-onset progressive degenerative myopathy with pseudohypertrophy of the calves, associated with highly elevated creatine kinase levels in blood and the almost complete absence (<2%) of dystrophin in muscle cells. Although dystrophin is detectable in patients with the milder Becker phenotype, the protein is less functional and is usually of abnormal size (3 ).