Classic endocrine therapy drugs for breast cancer include selective estrogen receptor modulators, such as tamoxifen and toremifene, which can compete with estrogen receptors to inhibit the effects of estrogen, thereby reducing the proliferation of breast cancer cells.491 For postmenopausal patients, aromatase inhibitors such as anastrozole, letrozole, and exemestane may be preferentially considered. This evidence concerns the gene CYP19A1 and breast cancer.