JAK1 and JAK2 are required for the activation of STAT6 in response to IL-4 in the cells that express IL-13Rα′ and IL-4Rβ chains of IL-4R. Wild type JAK1 (wtJAK1), JAK2 (wtJAK2), or both, and kinase-deficient JAK1 (▵JAK1), JAK2 (▵JAK2), or both, were cotransfected with (A) the IL-4Rβ chain and the IL-13Rα′ (α′β) or (B) the IL-4Rβ chain and γ_c (βγ) as described in Materials and Methods. After 48 hours, the cells were stimulated by IL-4 (50 ng/mL) and lysed in lysing buffer. Fifty micrograms of sample proteins were incubated for 20 minutes at room temperature with 0.5 to 1 ng of ³²P-labeled SBE1 probe. The samples were loaded on a 4% nonreducing polyacrylamide gel and run at 150 V for 2 hours. The gel was dried and analyzed by autoradiography.