We hypothesize that clonal evolution during bortezomib treatment can lead to Xc- and GSH-mediated bortezomib resistance in vivo. We investigated whether xCT mRNA levels could be used as a biomarker for bortezomib sensitivity in myeloma patients but no clear correlation was found for the 20 patients analyzed. The gene discussed is SLC7A11; the disease is plasma cell myeloma.