P450 enzymes constitute a family of monooxygenase enzymes that are involved in the metabolism of a wide array of endogenous and xenobiotic compounds including cholesterol. CYP7A1, is the rate limiting enzyme of bile acid synthesis in the liver, and its expression is mediated by the bile acid receptor FXR.CYP7B1 (oxysterol 7-(alpha)-hydroxylase) functions as an enzyme in the alternate bile acid synthesis pathway. Specifically, CYP7B1 metabolizes 25- and 27-hydroxycholesterol. The gene encoding human CYP7B1 maps to chromosome 8q21.3. Mutations in the CYP7B1 gene may cause a metabolic defect in bile acid synthesis characterized by elevated urinary bile acid excretion, severe cholestasis, cirrhosis and liver synthetic failure.