SIRT1 and hyperinsulinemic hypoglycemia, familial, 4: To determine whether the improvement in skeletal muscle mass reduction caused by 1,25(OH)2D deficiency by Sirt1 overexpression in MSCs is related to changes in muscle fiber cross-sectional area and type II fiber number, we collected tibialis anterior muscle from 2-month-old male wild-type, 1α(OH)ase−/− and Sirt1Tg1α(OH)ase−/− littermates.