During the 1980s and 90s many aromatase inhibitors were developed for potential use in treating breast cancer, and three eventually emerged for widespread use in both advanced and early ER-positive breast cancer (Arimidex) [34–36], exemestane (Aromasin) [37, 38], and letrozole (Femara) [39], but only anastrozole and exemestane have been used for cancer prevention. The gene discussed is CYP19A1; the disease is cancer.