DMD and Becker muscular dystrophy: Dystrophinopathies are related to the absence (Duchenne muscular dystrophy, DMD) or to the partial deficiency (Becker muscular dystrophy, BMD) of the dystrophin protein, encoded by the DMD gene on chromosome X. Although dystrophin is mainly expressed in the skeletal muscle, different isoforms are also expressed in other tissues, including the brain.