Primary mouse skeletal muscle cells (from Barx +/− animals, and between 14 and 16 passage doubling) were isolated as previously described (Rando and Blau, 1994 ; Meech et al., 2012 (link)) and cultured on MatTek dishes treated as described above for C2C12 cells. Myoblasts were cultured in Growth Medium (40% Ham’s F-10 (Sigma-Aldrich) and DMEM supplemented with 20% FBS, 5% Fungizone (Invitrogen), 1% penicillin/streptomycin, and 2.5 ng FGF (Fibroblast Growth Factor, Basic Human (Cat #: G507A, Promega; Madison, WI)) in 5% CO2 at 37°C. The Growth Medium was removed after 2–3 days and the same Differentiation Medium used for the C2C12 cells was added to the cultures to promote myotube formation.