ESR1 and breast cancer: In terms of the absence or presence of one or more of the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) and the Ki-67 status, breast cancers are classified into triple-negative breast cancers (ER−, PR−, HER2−), luminal A (ER+, PR+, HER2−, Ki-67low), luminal B (ER+, PRlow, HER2−, Ki-67high) and HER2+ (ER−, PR−, HER2high/overexpressed) subtypes [13].