ESR1 and breast cancer: As a heterogeneous disease, breast cancer is commonly classified into three subtypes based on receptor expression in clinical settings: luminal estrogen receptor (ER) and progesterone receptor (PR)-positive breast cancer, human epidermal growth factor receptor 2 (HER2)-positive breast cancer, and triple-negative breast cancer (TNBC) (ER−, PR−, and HER2−) [8, 9].