MBNL1 and myotonic dystrophy type 1: Since the upregulation of MBNL1 is a promising therapeutic target of DM1, and the level of circulating 25(OH)D (the precursor of calcitriol) correlates inversely with the disease severity in DM1 patients [15, 16], we started to address whether calcitriol regulates MBNL1 expression using C2C12 cells.