Cytochrome P450 17A1 (steroid 17α-monooxygenase, 17α-hydroxylase, 17-alpha-hydroxylase, 17,20-lyase, 17,20-desmolase) is an enzyme of the hydroxylase type that in humans is encoded by the CYP17A1 gene on chromosome 10. It is ubiquitously expressed in many tissues and cell types, including the zona reticularis and zona fasciculata of the adrenal cortex as well as gonadal tissues. CYP17A1 is a member of the cytochrome P450 superfamily of enzymes localized in the endoplasmic reticulum. Proteins in this family are monooxygenases that catalyze synthesis of cholesterol, steroids and other lipids and are involved in drug metabolism. CYP17A1 has both 17α-hydroxylase activity (EC 1.14.14.19) and 17,20-lyase activity (EC 1.14.14.32). The 17α-hydroxylase activity of CYP17A1 is required for the generation of glucocorticoids such as cortisol, but both the hydroxylase and 17,20-lyase activities of CYP17A1 are required for the production of androgenic and oestrogenic sex steroids by converting 17α-hydroxypregnenolone to dehydroepiandrosterone (DHEA).