While utrophin-deficient mice are healthy and generally show no signs of weakness [84], utrophin/dystrophin-deficient mice exhibit a more severe dystrophic phenotype than single Dmdmdx mutant mice [85]: i. e., the double knockout strain exhibits a phenotype that more closely resembles DMD in humans with severe progressive muscular dystrophy resulting in premature death. This evidence concerns the gene UTRN and Duchenne muscular dystrophy.