The purpose of this review is to analyze the clinical applications of a remarkable series of advances made in molecular genetics, primarily with regard to Becker muscular dystrophy. A new classification is required to clarify such syndromes as Duchenne and Becker muscular dystrophy. Dystrophinopathies can be seen in patients with early onset and a severe course (Duchenne muscular dystrophy), patients with later onset and milder weakness (Becker muscular dystrophy), patients with myalgia and cramp syndrome, and patients with dilated cardiomyopathies. Dystrophin testing in muscle is the most sensitive test for identification of dystrophinopathy patients, although gene deletion studies can make the diagnosis in most cases.