ESR1 and breast cancer: Since then luminal A (estrogen receptor (ER)-positive, progesterone receptor (PR)-positive, and human epidermal growth factor receptor (HER)-2/neu-negative) and basal-like (ER-negative, PR-negative, HER-2/neu-negative, and cytokeratin 5/6-positive and/or HER-1 positive) breast cancers have been widely studied clinically and epidemiologically [2-14], with luminal A cancers being of interest because they represent the largest percentage (45%) of cancers, and basal-like cancers, whereas rarer (5 to 15% of cases), having the poorest survival outcomes [4,15,16].