RYR1 and Duchenne muscular dystrophy: To confirm that RYR1 is functional in DMD myotubes, after Fura-2 loading (a dye to measure intracellular Ca2+), we stimulated myotubes with 100 μM 4-chloro-meta-cresol (4-CmC) in Ca2+-free medium to induce RYR1-mediated SR Ca2+ release.