ESR1 and breast cancer: Endocrine therapy that comprises the third generation of aromatase inhibitors (AIs)—Exemestane (Exe), Anastrozole (Ana) and Letrozole (Let)—as well as the selective estrogen receptor modulator (SERM) Tamoxifen and the selective estrogen receptor degrader (SERD) Fulvestrant have been used as the first-line therapy for both post- and pre-menopausal women with estrogen receptor-positive (ER+) breast cancer in early and advanced stages [1,2,3].