AbstractAromatase, a cytochrome P-450, catalyzes the formation of aromatic C18 steroids from C19 androgens. The conversion reaction consists of complex processes which involve oxidative removal of the angular C-19 methyl group as formic acid and subsequent aromatization of the A ring (Fig. 1). Three moles each of NADPH and O2are consumed for each mole of estrogen formed. This enzyme has received considerable attention because of the central importance of estrogens in many reproductive and metabolic processes. A more detailed understanding of the role of this enzyme in steroidgenic metabolism has necessitated characterization of the protein at a molecular level.