Schematic showing the details of the estrogen pathway. Androstenedione and testosterone are converted to estrone (E1) and 17β-estradiol (E2) by aromatase, also named CYP19A1, as the key enzyme of estrogen biosynthesis. Aromatase can be inhibited by letrozole, resulting in blocked synthesis of the estrogens E1 and E2. By contrast, cytochrome P4501A1 (CYP1A1), as an enzyme involved in estrogen catabolism, converts estrone into the hydroxylated catechol estrogen 2-OH-E1. 2-OH-E2: 2-hydroxyestradiol; 4-OH-E1: 4-hydroxyestrone; 4-OH-E2: 4-hydroxyestradiol; COMT: Catechol-O-methyltransferase; DHEA: Dehydroepiandrosterone; ER: Estrogen receptor; HSD3B1: hydroxysteroid 3β-dehydrogenase 1. The diagram is modified from a figure published previously by Dumas and Diorio .