ESR1 and breast cancer: According to the expression of estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and Ki-67 labelling index (Ki-67) which reflects the proliferation [7], breast cancer has four primary molecular subtypes, namely luminal A, luminal B, HER2-positive, and triple-negative breast cancer (TNBC) [5,8].