ESR1 and breast cancer: Depending on the presence or absence of estrogen receptor (ER), human epidermal growth factor receptor 2 (HER2) and progesterone receptor (PR), breast cancer is clinically classified into luminal A (ER+/HER−/PR+), luminal B (ER+/HER+/PR−), HER2 positive (ER−/HER+/PR−), and basal-like (ER−/HER−/PR−) to guide the hormone therapy targeting the present receptor.