ESR1 and breast neoplasm: Breast tumors are generally divided into three main subtypes based on hormone status: estrogen receptor (ER), human epidermal growth factor receptor 2, and HER2 (HER2) [2,3], together with some rare subtypes, including luminal subtypes that express ER and/or progesterone receptor (PR), HER2-positive subtypes that overexpress HER2, and triple-negative breast cancer (TNBC) [2,4].