ESR1 and breast cancer: Based on the presence of the expression of these receptors examined by immunohistochemistry, BC can be divided into four basic subtypes that correlate with mRNA intrinsic subtypes-Luminal A (ER+, PR≥20%, HER2−, Ki67 < 20%), Luminal B (ER+, PR < 20% and/or HER2+ and/or Ki67≥ 20%), HER2 subtype (ER−, PR−, HER2+), basal-like (triple-negative: ER−, PR−, HER2−) [6].