ESR1 and breast cancer: Through the combinations of molecular markers’ expression in the cancer cells, such as estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2), it is possible to define principal intrinsic BC subtypes: luminal A, luminal B, HER2-enriched, basal-like/triple-negative and normal-like, which are characterized by different pathophysiology, prognosis and sensitivity to treatments [2,3].