ESR1 and breast cancer: By studying the activity of BER against human breast cancer cells and dissecting the molecular mechanisms of action of BER, we have demonstrated that BER significantly suppresses the in vitro and ex vivo growth of highly metastatic human breast cancer cells and enhances anticancer activity of anticancer agents celecoxib, trichostatin A and carmofur against the growth of MDA-MB-231 cells; BER also displays the strong activity of inducing apoptosis in both estrogen receptor-negative MDA-MB-231 cells and estrogen receptor-alpha-positive MCF-7 breast cancer cells.