Effective strategies to treat ER-positive BC include endocrine agents that compete with estrogen for binding to its receptor, such as selective estrogen-receptor modulators (SERMs) and antiestrogens or reducing the levels of circulating estrogens by the administration of agents such as third-generation aromatase inhibitors (AIs), which have been shown to be more effective than tamoxifen in postmenopausal women in neoadjuvant and adjuvant settings [9]. The gene discussed is ESR1; the disease is breast cancer.