ESR1 and breast cancer: It has been reported that endoxifen reduces ERα protein expression in breast cancer cell lines (e.g., MCF-7, T47D, and Hs578t/ERα) at the concentrations of 10 to 1000 nM, whereas it blocks ERα-mediated transcriptional activation and inhibits estrogen-induced breast cancer cell proliferation at higher concentrations (≥100 nM) [24].