ESR1 and breast cancer: Based on the expression status of hormone receptors (HRs) (estrogen receptor, ER, and progesterone receptor, PR) and human epidermal growth factor receptor 2 (HER2), breast cancer is divided into luminal A (HR+/HER2−, 69%), luminal B (HR+/HER2+, 10%), HER2-enriched (HR−/HER2+, 8%) and triple-negative breast cancers (TNBC) (HR−/HER2−, 13%) [42,43].