ESR1 and breast cancer: They were further tested for their antiproliferativeactivity in two human breast cancer cell lines, namely, MDA-MB 231and MCF-7 cells, which differ in the expression of estrogen (ER)/progesterone(PR) receptors and human epidermal growth factor receptor type 2.While MDA-MB 231 cells are considered triple negative since they lackall three receptor types, MCF-7 cells are hormone-dependent (ER+/PR+).