Vitamin D3, also called cholecalciferol, is one of the forms of vitamin D. Vitamin D3 is a steroid hormone that has long been known for its important role in regulating body levels of calcium and phosphorus, in mineralization of bone, and for the assimilation of Vitamin A. It is structurally similar to steroids such as testosterone, cholesterol, and cortisol (although vitamin D3, itself, is a secosteroid). Vitamin D3 is a derivative of 7-dehydroxycholesterol formed by ultraviolet rays breaking the C9-C10 bond. It differs from ergocalciferol in having a single bond between C22 and C23 and lacking a methyl group at C24. Vitamin D3 can also come from dietary sources, such as beef liver, cheese, egg yolks, and fatty fish (PubChem). The first step involved in the activation of vitamin D3 is a 25-hydroxylation catalyzed by 25-hydroxylase in the liver and then by other enzymes. The mitochondrial sterol 27-hydroxylase catalyzes the first reaction in the oxidation of the side chain of sterol intermediates. The active form of vitamin D3 (calcitriol) binds to intracellular receptors that then function as transcription factors to modulate gene expression. Like the receptors for other steroid hormones and thyroid hormones, the vitamin D receptor has hormone-binding and DNA-binding domains. The vitamin D receptor forms a complex with another intracellular receptor, the retinoid-X receptor, and that heterodimer is what binds to DNA. In most cases studied, the effect is to activate transcription, but situations are also known in which vitamin D suppresses transcription. Calcitriol increases the serum calcium concentrations by (1) increasing GI absorption of phosphorus and calcium, (2) increasing osteoclastic resorption, and (3) increasing distal renal tubular reabsorption of calcium. Calcitriol appears to promote intestinal absorption of calcium through binding to the vitamin D receptor in the mucosal cytoplasm of the intestine. Subsequently, calcium is absorbed through the formation of a calcium-binding protein.