CYP2R1 is a member of the cytochrome P450 superfamily of enzymes. The cytochrome P450 proteins are mono-oxygenases which catalyze many reactions involved in drug metabolism and synthesis of cholesterol, steroids and other lipids. This enzyme is a microsomal vitamin D hydroxylase that converts vitamin D into the active ligand for the vitamin D receptor. Defects in CYP2R1 are a cause of 25-hydroxyvitamin D(3) deficiency, also known as pseudovitamin D(3) deficiency rickets due to 25-hydroxylase deficiency. First described in patients who had rickets at a young age despite a history of adequate vitamin D intake. The patients sera had low calcium concentrations, low phosphate concentrations, elevated alkaline phosphatase activity and low levels of 25-hydroxyvitamin D.