Vitamin D receptors （VDRs） belong to the NR1I family， which also includes pregnane X （PXR） and constitutive androstane （CAR） receptors which form heterodimers with members of the retinoid X receptor family. VDR is expressed in the intestine， thyroid and kidney and has an imperative role in calcium homeostasis. VDRs inhibit expression of 1alpha-hydroxylase （the proximal activator of 1，25（OH）2D3 and induce expression of the 1，25（OH）2D3 inactivating enzyme CYP24. Additionally， VDR has recently been recognized as an additional bile acid receptor alongside FXR and may function to protect gut against the toxic and carcinogenic effects of these endobiotics. Hereditary mutations in the VDR gene leads to rickets， which is typified by muscle weakness， growth retardation， bone deformity and secondary hyperparathyroidism. The human gene encoding the VDR is localized to chromosome 12q12-q14.
