Phosphospecificity Whole cell extracts of control (1), EGF stim ulated (2) or pervanadate treated (3) A 431 tumor cells were applied to SDS-PAGE (ca 20.000 cells per lane) and transferred to a PVDF membrane. The immunoblot was probed with mab STAT3-23G5 (0.5 ug/ ml) for 1h at RT and developed by ECL (exp. time: 0 sec).