XBP1 and breast cancer: Therefore, the pathways characterized by the 34 genes that are specifically up‐regulated in the aged female‐derived mammary tumors are consistent with the ability of the ERα to promote both the XBP‐1 axes of the endoplasmic reticulum UPR (UPRER) and the ERα axis of the mitochondrial UPR (UPRmt), in addition to the up‐regulation of the anti‐apoptotic response through MCL‐1.