Recent advances in molecular biology have indicated that many mutant animal models of muscular dystrophy share common genetic and protein abnormalities similar to those of the human disease. The best example is a model of Duchenne muscular dystrophy (DMD), the mdx mouse. Similar to dystrophic muscle in DMD patients, dystrophin protein is not expressed along the surface membrane, even though the mdx mouse has no apparent signs of muscular dysfunction. Because clinical and pathologic findings in the dystrophic (mxd) dog are similar to those in DMD patients, it also has been regarded as a good model for therapeutic trials. The best known and most extensively studied dy+dy+ mouse lacks merosin (laminin alpha2), which is one subunit of a basement membrane protein, laminin. Because approximately half of all patients with the classical form of congenital muscular dystrophy also lack merosin, availability of this animal has revived interest in the study of the pathologic mechanism of fiber necrosis resulting from this membrane defect. The dystrophic hamster is a model of limb-girdle muscular dystrophy with sarcoglycan deficiency in which one of the dystrophin-associated glycoproteins, delta-sarcoglycan, is defective. Because these animal models have common protein and genetic defects similar to those seen in people with muscular dystrophies, they have been widely used to examine the effectiveness of gene therapy and the administration of pharmacologic and trophic factors.