ESR1 and breast cancer: In ER-negative/PR-negative MDA-MB-231 breast cancer cells, mifepristone and 4-hydroxytamoxifen potentiated one another's effect in inducing apoptosis associated with DNA fragmentation and cytochrome c release from the mitochondrial compartment and activation of downstream executer of apoptosis, caspase 3, mediated via upregulation of TGFβ1 (Liang et al. 2003).