NIK regulates the synergistic induction of STAT3 tyrosine phosphorylation by the TCR and IL-6R signals. NIK^+/+ and NIK^−/− naive CD4 T cells were stimulated for the indicated times with IL-6 (20 ng/mL), anti-CD3/CD28 (2 μg/mL), or IL-6 plus anti-CD3/CD28. Site-specific tyrosine phosphorylation of STAT3 (Tyrosine 705) was analyzed by flow cytometry. Data are representative of 3 experiments.