ER stress-induced activation of the endoribonuclease domain of the ER transmembrane protein, IRE1, splices an intron from precursor XBP1 mRNA (XBP1u) that results in translation of the active transcription factor XBP1s [20, 21], levels of which have been shown to correlate with sensitivity to bortezomib in myeloma patients [22]. This evidence concerns the gene XBP1 and plasma cell myeloma.