BHPI is a potent inhibitor of nuclear estrogen–ERα-regulated gene expression. Elicits sustained ERα-dependent activation of the endoplasmic reticulum (EnR) stress sensor, the unfolded protein response (UPR), and persistent inhibition of protein synthesis. IC50 value: Target: ERα inhibitor BHPI is effective because it elicits sustained ERα-dependent activation of the endoplasmic reticulum (EnR) stress sensor, the unfolded protein response (UPR), and persistent inhibition of protein synthesis. In ERα(+) cancer cells, BHPI rapidly hyperactivates plasma membrane PLCγ, generating inositol 1,4,5-triphosphate (IP3), which opens EnR IP3R calcium channels, rapidly depleting EnR Ca(2+) stores. BHPI distorts a newly described action of estrogen-ERα: mild and transient UPR activation. In contrast, BHPI elicits massive and sustained UPR activation, converting the UPR from protective to toxic.