XBP1 and plasma cell myeloma: Ixazomib, another proteasome inhibitor, promoted osteoblast formation and activity of osteoprogenitor cells from multiple myeloma patients through the activation of TCF/β-catenin signaling, unfolded protein response via IRE1α–XBP1 pathway, and upregulation of BMP2 in mesenchymal stromal cell (MSC) precursors [30,31].