English: Often used as a cancer treatment in postmenopausal women, aromatase inhibitors work by blocking the conversion of androstenedione and testosterone into estrone and estradiol, respectively, which are both crucial to the growth of developing breast cancers (aromatase inhibitors are also effective at treating ovarian cancer, but less commonly so). In the diagram, the adrenal gland (1) releases androstenedione (3) while the ovaries (2) secrete testosterone (4). Both hormones travel to peripheral tissues or a breast cell (5), where they would be converted into estrone (8) or estradiol (9) if not for aromatase inhibitors (7), which prevent the enzyme CYP19AI (also known as aromatase or estrogen synthase) (6) from catalyzing the reaction that turns androstenedione and testosterone into estrone and estradiol. In the diagram, Part A represents the successful conversion of androstenedione and testosterone into estrone and estradiol in the liver. Part B represents the blockage of this conversion by aromatase inhibitors both in peripheral tissues and in the breast tumor itself.