SILAC phosphoproteomic analyses and Western blotting revealed an increase of phosphorylation of stathmin at Ser38 and the unchanged steady-state stathmin in U266 cells upon proteasome inhibition (Fig. 2 ). It has been reported that the functional alteration of stathmin resulting from specific phosphorylation events may be involved in the process of apoptosis induced by proteasome inhibitors in proliferating cells [25] . To elucidate the differential phosphorylation of stathmin isoforms, Western blotting was performed to analyze the phosphorylation pattern of stathmin in bortezomib-treated U266 cells using stathmin antibody and specific phospho-antibodies against three known phospho-sites (Ser16, Ser25, Ser38) [26] . As shown in Figure 4A , in accordance with SILAC results, phosphorylation of stathmin at Ser38 was increased whereas steady-state stathmin remained unchanged. Furthermore, phosphorylation of stathmin at Ser16 was increased whereas Ser25 was decreased after bortezomib treatment (Fig. 4A ).