SPEG and myopathy: In contrast, MCK-Cre+Spegfl/fl mice11 (which we designate as Speg-KO mice) display a >95% decrease in both Spegβ and Spegα in both skeletal muscle (Fig. 1b) and heart (Fig. 1d) and display a life-threatening myopathy where both skeletal and cardiac muscle are severely affected.