The Duchenne type of muscular dystrophy is the most common of the childhood dystrophies. It is caused by an X-linked recessive gene and affects about one in 5,000 boys. The muscle weakness progresses and the child is usually wheelchair-bound by the age of eight to ten. Cardiomyopathy is present in the majority of cases and one third to one half have some degree of mental retardation. One of the characteristic features that is used to diagnostically differentiate between Duchenne muscular dystrophy and other types of childhood dystrophies is a grossly elevated level of creatine phosphokinase (CPK) in the blood serum. The level of CPK in serum is also used to determine the carrier state in females. As with most carrier tests for X-linked disorders, it is not a very exact one, since many carriers test within normal limits. The problem with lyonization is present in most X-linked diseases in which the product measured is not the primary gene product.