Vitamin D3 (cholecalciferol), synthesised in human skin by ultraviolet radiation action on 7-dehydrocholesterol, does not possess any biological activity. Vitamin D hormonal activity requires hydroxylation at the 25 and 1-alpha positions by cytochrome P450 enzymes CYP2R1 and CYP27B1 respectively. Vitamin D 25-hydroxylase (CYP2R1) catalyses the hydroxylation of vitamin D3 to calcidiol (CDL). Subsequent 1-alpha-hydroxylation of CDL by CYP27B1 produces calcitriol (CTL). CTL binds and activates the nuclear vitamin D receptor, with subsequent regulation of physiologic events such as calcium homeostasis, cellular differentiation and proliferation.Defects in CYP27B1 can cause rickets, vitamin D-dependent 1A (VDDR1A; MIM:264700), a disorder caused by deficiency of the active form of vitamin D (CTL) resulting in defective bone mineralization and clinical features of rickets. To date, 47 mutations have been identified, the majority of them (28) being missense mutations (Kim 2011, Cui et al. 2012).