ESR1 and breast cancer: While both MCF-7 and MDA-MB-231 cells expressed appreciable amounts of TNK2, MCF-7 cells are oestrogen-responsive breast cancer cells and have low levels of EGFR while MDA-MB-231 breast cancer cells do not express oestrogen receptor alpha, progesterone receptor or human epidermal growth factor receptor 2 but have very high levels of EGFR.