CYP2R1 catalyzes conversion of vitamin D_3 to 25-hydroxyvitamin D_3 (25OHD3), while CYP27B1 catalyzes conversion of 25OHD_3 to 1α, 25-dihydroxyvitamin D_3 (1,25(OH)2D3), the active form of vitamin D_3 On the contrary, CYP24A1 inactivates 1,25(OH)2D3 by multi-step monooxygenation reactions. More than 30 CYP27B1 mutations containing missense and nonsense mutations have been found in vitaminD-dependent rickets type I (VDDR1) patients, while 3 types of CYP2R1 mutation were found in VDDR1 patients. Recently, more than 20 CYP24A1 mutations were found, and a clear relationship was revealed between CYP24A1 mutations and idiopathic infantile hypercalcemia. These findings clearly demonstrate that deficiency of one of the three CYPs could cause hypo- or hypervitaminnosis D, resulting in disruption of calcium homeostasis. Recently, several reports on CYP2R1 and/or CYP27B1 genes polymorphisms and susceptibility to type I diabetes were published. GG genotype of (G>A)(rs10741657) CYP2R1 polymorphism and CC genotype of -1260C>A (rs10877012) CYP27B1 polymorphism increased the risk of type I diabetes. In the immune system, 1,25(OH)2D3 has been shown to suppress Th1-type immune responses. Low levels of 25OHD3 and/or 1,25(OH)2D3 in GG type of CYP2R1 and CC type of CYP27B1 may explain the excessive Th1-type responses to result in an autoimmune disease, type I diabetes.