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The mdx mouse, a mouse model of Duchenne muscular dystrophy, carries a loss-of-function mutation in dystrophin, a component of the membrane-associated dystrophin–glycoprotein complex. Unlike humans, mdx mice rarely display cardiac abnormalities and exhibit dystrophic changes only in a small number of heavily used skeletal muscle groups. By contrast, mdx:MyoD^−/− mice lacking dystrophin and the skeletal muscle-specific bHLH transcription factor MyoD display a severe skeletal myopathy leading to widespread dystrophic changes in skeletal muscle and premature death around 1 year of age. The severely increased phenotype of mdx:MyoD^−/− muscle is a consequence of impaired muscle regeneration caused by enhanced satellite cell self-renewal. Here we report that mdx:MyoD^−/− mice developed a severe cardiac myopathy with areas of necrosis associated with hypertrophied myocytes. Moreover …