Jak3 is not tyrosine phosphorylated by IL-12 in T cells activated with PHA + IFN-γ. PBMC cultured with either PHA alone or PHA + IFN-γ were stimulated for 20 minutes with either medium alone (lanes 1 and 4), IL-12 (lanes 2 and 5), or IL-2 (lanes 3 and 6). Immunoprecipitations were performed on whole cell lysates (30 to 40 × 10⁶ cells/lane) with an antibody to Jak3, followed by a Western blot with an antibody to phosphotyrosine. Results are representative of two separate experiments.