An adrenal microsomal cytochrome P450 enzyme that catalyzes the 21-hydroxylation of steroids in the presence of molecular oxygen and nadph-ferrihemoprotein reductase. This enzyme, encoded by CYP21 gene, converts progesterones to precursors of adrenal steroid hormones (corticosterone; hydrocortisone). Defects in CYP21 cause congenital adrenal hyperplasia (adrenal hyperplasia, congenital).