Steroid 21-hydroxylase, also called steroid 21-monooxygenase,[1] 21α-hydroxylase,[2] and, less commonly, 21β-hydroxylase,[3][4] is an enzyme that hydroxylates steroids at the C21 position[5][6] and is involved in biosynthesis of aldosterone and cortisol. The enzyme converts progesterone and 17α-hydroxyprogesterone into 11-deoxycorticosterone and 11-deoxycortisol, respectively.[7][8] The products of the conversions then continue through their appropriate pathways towards creation of aldosterone and cortisol.