IL-21 induces tyrosine phosphorylation of JAK-1 and JAK-3 and of STAT-1, STAT-3, and STAT-5, but not of JAK-2 upon ligand binding. CLL B cells were stimulated with rhIL-21 (80 ng/mL) for the indicated time intervals, lysed in appropriate buffers, and analyzed by Western blotting using mAbs specific for phosphorylated or unphosphorylated forms of signal transducer proteins, or for β-actin.