The plasmids were introduced into U266 cells using the X005 mode of Nucleofector (Amaxa, Cologne, Germany), according to the Optimized Protocol for the U266B1 cell line. Electroporated cells were cultured in medium with the presence of 0.5 mg/ml G418 (Mediatech, Manassas, VA) for 14 days, and then cultured in the 96-well plates for dilution cloning. Finally, the clone selected from pReceiver-M01 blank vector transfected U266 cells was designated as U266-NC. The clones selected from wild type or mutant stathmin plasmids transfected U266 cells were designated as U266-WT, U266-S16A, U266-S25A or U266-S38A, respectively.