To investigate this effect along with the underlying mechanisms at the molecular level, the estrogen receptor-negative (ER-), progesterone receptor-negative (PR-), and HER2-negative (HER2-) MDA-MB-231 human breast cancer cells and the ER+, PR+, and HER2-MCF-7 human breast cancer cells were selected for this study. This evidence concerns the gene ESR1 and breast cancer.