ESR1 and breast cancer: Depending on the presence or absence of these molecular markers, there are four principal types of breast cancer: HER2+ subtype, non-luminal (characterized by ER+ and/or PR+, HER2+), HER2+ subtype, luminal (characterized by ER−, PR−, HER2+), luminal A-like subtype (ER+ and/or PR+, HER2−, Ki-67 < 14%), luminal B-like subtype (ER+ and/or PR+, HER2+ or HER2−, Ki-67 > 14%), and basal-like subtype (triple-negative breast cancer, TNBC) (ER−, PR−, HER2−) [3,4].