BC cell lines with diverse expression of estrogen receptor (ER), progesterone receptors (PR), and human epidermal growth factor receptor-2 (HER2) were used and included MCF-7 and T47D (ER+/PR+/HER2−), MDA-MB-453 (ER−/PR−/HER2+), MDA-MB-231, and MDA-MB-468 (ER−/PR−/HER2−) cells. This evidence concerns the gene ESR1 and breast cancer.