CYP19A1 and breast cancer: AIs inhibit a key enzyme for the conversion of androgensto estrogens.6,7 The 4-hydroxy metabolite of tamoxifencompetitively binds to ER in BC cells, thereby inhibiting transcriptionand ensuing mitogenic effects in both pre- and postmenopausal women.8,9 AIs suppress aromatase activity, thus, decreasing circulating estrogenlevels and preventing BC cells from proliferation.