The vitamin D endocrine system is based in the proximal convoluted tubule cells of the kidney. The key enzyme in this system is the 25-hydroxy-1α-hydroxylase or CYP27B1. It is tightly regulated primarily by the parathyroid hormone and 1α,25-dihydroxyvitamin D3 itself. It produces the vitamin D hormone from the circulating form of vitamin D, i.e. 25-hydroxyvitamin D3 by 1α-hydroxylation. The hormone then activates the vitamin D receptor in target organs such as intestinal enterocytes, osteoblasts of bone and renal tubule cells. The activated receptor next stimulates transcription or suppresses transcription of target genes, thus regulating cellular proteins that carry out a variety of functions.