We found that both fetal SERCA1b Δex22 and fetal Cav1.1 Δex29 were significantly increased in T15 DM1 myotubes as compared to controls, where the proportion of adult isoforms of SERCA (ex22) and Cav1.1 (ex29) increased with the progression of the in vitro muscle maturation. The gene discussed is CACNA1S; the disease is myotonic dystrophy type 1.