Wild type (wt) C57BL/6 mice, dystrophin-deficient mdx,23 and dystrophin/utrophin-deficient double mutant mdx-utr24,25 mice, backcrossed on the C57BL/6 background for more than 12 generations, were used for the study. Details about the mutations are described in our earlier work.33 Through the text, the dystrophin−/− status (utrophin+/+) is named “mdx,“ and double mutant mice (dystrophin−/− and utrophin−/−) are termed “mdx-utr.” The mice were genotyped using standard PCR-assays. For comparisons between mdx and mdx-utr, littermates were used.