The mdx mouse has been used for the development of cellular and gene therapies for Duchenne muscular dystrophy. The relatively frequent occurrence of dystrophin-positive muscle cells called revertants has hampered these efforts by interfering with data interpretation. The mdx4cvand mdx5cvdystrophin mouse mutants have approximately 10-fold fewer revertants than the mdx mutant at both 2 and 6 mo. The mdx3cvdystrophin mouse mutant may be a useful model for some types of human dystrophin deficiencies in which the levels of dystrophin are low but not completely absent.