Becker muscular dystrophy: A chronic, progressive muscle degeneration disease. A form of muscular dystrophy that is quite similar to Duchenne muscular dystrophy, except that patients with Becker do produce some of the key protein, dystrophin, whereas those with Duchenne do not. Progression of the disease in Becker type is slower than in Duchenne, and symptoms may appear as late as the mid-twenties.

Becker muscular dystrophy is a mild version of Duchenne. Both diseases result from mutations in the huge gene in region Xp21.2 on the X chromosome that encodes dystrophin. (The other principal form of X-linked, late-onset muscular dystrophy is Emery-Dreifuss muscular dystrophy, which results from mutation in the gene encoding emerin on Xq28.)