A cytochrome P450 monooxygenase that plays a major role in adrenal steroidogenesis. Catalyzes the hydroxylation at C-21 of progesterone and 17alpha-hydroxyprogesterone to respectively form 11-deoxycorticosterone and 11-deoxycortisol, intermediate metabolites in the biosynthetic pathway of mineralocorticoids and glucocorticoids (PubMed:25855791, PubMed:10602386, PubMed:16984992, PubMed:22014889, PubMed:27721825). Mechanistically, uses molecular oxygen inserting one oxygen atom into a substrate, and reducing the second into a water molecule, with two electrons provided by NADPH via cytochrome P450 reductase (CPR; NADPH-ferrihemoprotein reductase) (PubMed:25855791). O2 + progesterone + reduced [NADPH--hemoprotein reductase] = 21-hydroxyprogesterone + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] 17alpha-hydroxyprogesterone + O2 + reduced [NADPH--hemoprotein reductase] = 11-deoxycortisol + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] The leucine-rich hydrophobic amino acid N-terminal region probably helps to anchor the protein to the microsomal membrane. Seven non deleterious alleles are known: CYP21A2*1A, CYP21A2*1B, CYP21A2*2, CYP21A2*3, CYP21A2*4, CYP21A2*5 and CYP21A2*6. The sequence shown corresponds to allele CYP21A2*1B. Deleterious alleles are mostly generated by recombinations between CYP21A2 and the pseudogene CYP21A1P through gene conversion. This process consists of recombination events that either delete CYP21A2 or transfer deleterious mutations from CYP21A1P to CYP21A2. The disease is caused by variants affecting the gene represented in this entry. Belongs to the cytochrome P450 family. CYP21A2 alleles