To test the hypothesis that the Duchenne muscular dystrophy gene limits the proliferation of myoblasts, we studied myoblasts from a woman who was a carrier of the Duchenne gene and also heterozygous for glucose-6-phosphate dehydrogenase (G6PD), an X-linked enzyme. G6PD-A and G6PD-B cells did not differ in proliferative capacity, implying no difference in proliferative capacity of cells bearing the Duchenne gene or the normal allele.