NIK is required for differentiation of Th17 cells but is dispensable for the differentiation of other CD4 effector cells. (A) Naive CD4 T cells isolated from NIK^+/+ and NIK^−/− mice were stimulated for 72 hours with plate-bound anti-CD3 and anti-CD28 (1 μg/mL) under Th0, Th1, or Th2 conditions followed by flow cytometry to measure the frequency of IFN-γ producing Th1 cells and IL-4 producing Th2 cells. (B) Naive CD4 T cells were stimulated for 72 hours with plate-bound anti-CD3 and anti-CD28 (1 μg/mL) under Th0, Th17, or iTreg conditions and then subjected to flow cytometry to determine the frequency of Foxp3 producing iTregs and IL-17A producing Th17 cells. Data in both panels A and B are representative of 3 independent experiments.