Moreover, there were significantly fewer muscle fibers with non-peripheral nuclei in CASTB6F1 Col4a1+/Δex40 mice compared to C57BL/6J Col4a1+/Δex40 mice (p<0.05), indicating that the CAST/EiJ strain has one or more loci that can also ameliorate Col4a1-induced myopathy. This evidence concerns the gene COL4A1 and myopathy.