Hormonal therapies (HTs) for breast cancer aim to inhibit the ER (tamoxifen, other selective ER modulators, and the ER-degrader fulvestrant) or to suppress the production of estrogen in the body through ovarian ablation or suppression in premenopausal women, as well as aromatase inhibitors (AI) in postmenopausal women (Visovsky, 2014). The gene discussed is ESR1; the disease is breast cancer.