1alpha,25-dihydroxyvitamin D3 (vitamin D3) is a hormone that plays a crucial role in the regulation/metabolism of calcium and phosphorus in the small intestine, kidney and bone. In addition it is also involved in immune function, tumor suppression, growth regulation and parathyroid hormone secretion. VDR, the vitamin D3 receptor, exists as a heterodimer with RXR. Vitamin D3-bound VDR-RXR, along with other co-activator proteins, mediates the transcriptional regulation of a number of genes. Hence any disruption to the vitamin or its receptor would have consequences in a number of the key physiological processes listed above.