ESR1 and breast cancer: The triple-negative breast cancer (TNBC) MDA-MB-231 cells, which are highly aggressive, invasive, poorly differentiated, and lack the expression of estrogen receptors (ER), progesterone receptors (PR) and human epidermal growth factor receptor 2 (HER2) receptor; and due to these characteristics, these cells are widely used as a prototype of TBNC breast cancer [29].