However, over the past two decades aromatase inhibitors (AI), in particular the third-generation agents anastrozole, letrozole and exemestane, which prevent the aromatization of endogenous androgens into estrogen, have largely replaced tamoxifen as the treatment of choice for postmenopausal women with ER-positive breast cancer due to their superior effect on cancer recurrence, the development of cancer in the opposite breast, and breast cancer-related mortality as well as having an improved safety profile [11,12]. The gene discussed is CYP19A1; the disease is cancer.