ESR1 and breast cancer: As a heterogeneous tumor, breast cancer is frequently divided into the following molecular subtypes: Luminal A, Luminal B, human epidermal growth factor receptor 2 (HER2) overexpression, and triple-negative breast cancer (TNBC), according to the expression of estrogen receptor (ER), progesterone receptor (PR), HER2, and proliferation marker Ki67 [3].