ESR1 and breast cancer: The currently accepted biomarkers include the estrogen (ER), progesterone (PR) and human epidermal growth factor 2 (HER2) receptors [3], diving breast cancer into the following subtypes: luminal A (ER+, PR+/−, HER2−), luminal B (ER+, PR+/−, HER2+), HER2 (ER−, PR−, HER2+), basal-like (ER−, PR−, HER2−) and claudin-low (ER−, PR−, HER2−) [4, 5].