The functional significance of stathmin phosphorylation in the response of MM cells to bortezomib was then examined more rigorously. To this end, stable U266 cell clones overexpressing the stathmin wild-type (U266-WT) or mutants (U266-S16A, U266-S25A and U266-S38A) were generated using His-tagged constructs. Figure 5A shows the expression of His-tagged target proteins as well as stathmin expression in these cells. The complete methods and characterization in terms of proliferation, cell cycle, colony-forming efficiency (CFE) and apoptotic ratio of these cells are described in the Supplemental Data S1. We observed that all these cells had a comparable growth pattern and CFE, similar proportions of cells in G1, S, and G2 plus M phases and similar apoptotic ratio (Table S6) . However, there were significant differences between these cells with regard to their sensitivity to bortezomib treatment. As shown in Figure 5B , cells transfected with wild-type stathmin (U266-WT) exhibited significant increase in bortezomib-induced cell death compared with parental cells (U266). In contrast, cells transfected with mutant stathmin (U266-S16A, U266-S25A or U266-S38A) were significantly less sensitive to bortezomib lethality than U266 cells (P,0.05 for U266-S16A and U266-S38A or P,0.01 for U266-S25A, respectively) (Fig. 5B) .