ESR1 and breast cancer: Breast cancer (BC) is the most common type of gynecological cancer, and it is the highest cancer-related cause of death among them.1Oncological therapy includes surgery, radiotherapy and systemic therapy, as chemotherapy and hormone therapy.2Hormone therapy (HT), such as anastrozole and tamoxifen, is used for patients whose tumor is positive for estrogen receptor, which reaches 70% of BC cases.