| Component = | Process = | Orthologs = }} Steroid 21-hydroxylase (or steroid 21-monooxygenase or 21α-Hydroxylase; ) is a cytochrome P450 enzyme which is involved with the biosynthesis of the steroid hormones aldosterone and cortisol. The CAS number is 9029-68-9. 21-Hydroxylase is encoded by the gene CYP21B. Reaction It catalyses the hydroxylation of the carbon atom 21 in steroids (adding an "–OH"), which is necessary with the formation of these hormones. Image:Progesterone-2D-s… (More on 21-hydroxylase)