ESR1 and breast cancer: According to hormone receptors (HRs) (including estrogen receptor (ER) and progesterone receptor (PR)), human epidermal growth factor receptor 2 (HER2), and Ki67 (a proliferation index marker) status, breast cancer is classified into four major subgroups, including luminal A, luminal B, HER2-enriched (HER2 +), and triple-negative breast cancer (TNBC) subtypes [5].