ESR1 and breast cancer: It is well known that different combinations of the presence of estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (Her2) status, Ki67 protein and tumor grade define five basic molecular subtypes of breast cancer, luminal A, luminal B Her2+/luminal B Her2−, basal/triple-negative, normal-like, and Her2-enriched [8,9].