It has been reported that phosphorylation turns off the microtubule destabilizing activity of stathmin [27, 28] and that proteasome inhibitors increase tubulin polymerization and stabilization in myeloma cells [29] . We thus tested whether bortezomib actually induced changes in the polymerization status of the MTs in these cells. Indeed, when these cells were treated with bortezomib for 24 hours, we observed an increase in the amount of tubulin in the polymerized 'P' fraction as compared with untreated cells (Fig. 5C ). The baseline proportion of a-tubulin in the polymerized fraction ranged from ,43% to 52%, while the polymerized proportion observed after bortezomib treatment was ,60-90% (Table 1) .