XBP1 and plasma cell myeloma: STF083010 is an inhibitor of IRE1α endonuclease activity; which can block endogenous XBP1 mRNA splicing and displays cytostatic and cytotoxic effects in CD138+ multiple myeloma (MM) cells in vitro [14], STF083010 is also reported to inhibit bortezomib-induced XBP1 activity in myeloma xenografts in vivo but does not alter IRE1α kinase activity [24].