ET drugs either reduce E2 availability (e.g., aromatase inhibitors (AIs)) or block ERα activity by directly binding to the receptor; 4OH-tamoxifen (Tam), a selective ER modulator (SERM), inactivates ERα transcriptional activity, and fulvestrant (ICI182,780—ICI), a selective ER down-regulator (SERD), eliminates ERα from BC cells [11]. Here, ESR1 is linked to breast cancer.