XBP1 and breast carcinoma: Moreover, several studies linked estrogen receptor signaling to the regulation of the UPR [7,8]: the glucose regulated protein 78 (GRP78), also called binding immunoglobulin protein (BiP), and the x-box binding protein 1 (XBP1) are upregulated in endocrine- and chemotherapy-resistant breast cancers [7,9,10,11,12,13].