(A) Clonal assay from single myofiber explants cultured in growth medium (GM) for 6 days (n = 3 independent preparations). (B) Clonal assay from FACS-sorted SCs were cultured in GM for 7 days (n = 3 independent preparations). (C) Clonal assay from nontargeted and MKP-5 siRNA knockdown SCs isolated from C57BL/6J muscles. Cells were cultured in GM for 7 days (n = 3 independent preparations). (D) Gastrocnemius muscles were CTX injured for 24 hours and injected with BrdU; 18 hours later, cells were analyzed for BrdU and MYF5 expression (n = 5 per genotype). Photomicrographs show representative images of stained sections: MYF5 (pink); nuclei (blue); BrdU (green); nonspecific staining (red). Scale bar: 50 μm. Percentage of BrdU⁺/MYF5⁺ cells are quantified in E. (F) SCs were seeded at equal cell numbers to reach 90% confluence the next day, then the growth medium was switched to differentiation medium for 3 days. Cells were stained for myosin heavy chain (green) and nuclei (blue). Scale bar: 50 μm. (G) Fusion index was calculated as a percentage of nuclei in myotubes with greater than or equal to 2 nuclei divided by the total number of nuclei (n = 3 independent preparations). (H) SCs differentiated for 3 days after MKP-5 knockdown were stained for myosin heavy chain (green) and nuclei (blue). Scale bar: 50 μm. (I) Fusion index calculated as in G (n = 3 independent preparations). (A–I) All mice were 8-week-old males. For each independent preparation, 2 mice per genotype were used. Data represent the mean ± SEM. *P < 0.05 and **P < 0.01 compared with the Mkp5^+/+ or nontargeted siRNA–treated group.