– BHPI is a potent ERα inhibitor that blocks nuclear estrogen–ERα-regulated gene expression effectively. It triggers sustained ERα-dependent activation of the endoplasmic reticulum (EnR) stress sensor, notably the unfolded protein response (UPR), and consistently inhibits protein synthesis. With an IC50 value targeting ERα, BHPI’s mechanism involves the rapid hyperactivation of PLCγ on the plasma membrane in ERα(+) cancer cells, which produces inositol 1,4,5-triphosphate (IP3). This, in turn, opens EnR IP3R calcium channels, leading to a rapid depletion of EnR Ca(2+) stores. SIGNIFICANTLY, BHPI alters estrogen-ERα’s usual effect of causing mild and transient UPR activation by inducing a severe and prolonged UPR activation, transforming the UPR from a protective to a toxic response.