XBP1 and Alzheimer disease: The expression of several UPR transcriptional factors and mediators, such as C/EBP homologous protein (CHOP), binding immunoglobulin protein (BiP or GRP78), activating transcription factor 6 (ATF6), activating transcription factor 4 (ATF4), and X-box binding protein 1 spliced variant (XBP1spl) have been reportedly increased in AD brains [21,22,23,24,25,26,27].