The mRNA expression of nine previously described genes that may affect resistance to multiple myeloma (MM), viz., ABCB1, CXCR4, MAF, MARCKS, POMP, PSMB5, RPL5, TXN, and XBP1, was compared between bortezomib-refractory and bortezomib-sensitive patients. This evidence concerns the gene CXCR4 and plasma cell myeloma.