ESR1 and breast cancer: Endocrine therapies, including selective estrogen receptor modulators (SERMs, e.g., Tamoxifen), selective estrogen receptor down-regulators (e.g., Fulvestrant) and aromatase inhibitors (AIs, e.g., Letrozole, Anastrozole, Exemestane) currently represent the backbone of HR+ BC management both in the early and advanced setting.