ESR1 and breast adenocarcinoma: The cell line MCF-7 is human breast adenocarcinoma expressing estrogen receptor alpha (ER-α) [33], while the cell line MDA-MB-231 represents triple negative breast cancer (TNBC) adenocarcinoma and lacks ER, progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) [34].