ESR1 and breast cancer: And, finally, also no concordances with the third cytotoxic standard compared, tamoxifen, a member of the triphenylethylene group of compounds, which acts as a selective estrogen receptor modulator (SERM) used to treat estrogen receptor-positive (ER+) breast cancer because it acts competing with 17β-estradiol at its receptor site (MCF-7 is an ER+ cell line).