Effect of mutations on cytokine-induced nuclear localization of STAT3. (A) STAT3^−/− MEF cells were transiently transfected with Ac-GFP1–tagged WT or mutant STAT3 expression constructs and incubated without (left panels) or with (right panels) IL6/sIL6R. Cells were fixed, stained with DAPI, and examined by fluorescence microscopy; representative fields are shown. (B) The % FN in cells was calculated for 30 to 40 cells incubated with IL6/sIL6R and the mean ± SEM of 3 assays is shown (∗P < .005; Student t test).