ESR1 and breast cancer: According to estrogen and progesterone receptors (ER and PR) status, Ki67 proliferation index and HER2 expressions, BC are grouped in Luminal-A (ER+ and/or PR+, Ki67 < 20, HER2−), Luminal B- (ER+ and/or PR+, Ki67 ≥ 20, HER2−), Luminal B+ (ER+ and/or PR+, HER2+), HER2+ non-Luminal (ER−, PR−, HER2+), and triple-negative (ER−, PR−, HER2−).