Anti-estrogen therapy for BC currently includes (1) selective ER modulators (SERMs), (2) selective ER downregulators (SERDs), (3) aromatase inhibitors (AIs), and (4) suppression of ovarian function, which is performed using gonadotropin-releasing hormone agonists, or a combination of two or more of these strategies [34]. The gene discussed is ESR1; the disease is breast cancer.