Crk (Crk I and II) proteins and closely related CrkL are adapters which are commonly involved in various signaling processes in various cells, and these proteins share many ligands, Whether they have redundant or distinct physiologic roles is unclear. By coprecipitation and far Western blotting analysis, we demonstrate that Crk (I/II) binds to tyrosine phosphorylated STATE in cells stimulated by cytokines such as thrombopoietin (TPO) and interleukin-2 (IL-2), The association did not require nuclear elements and can be observed in primary cells as this was also demonstrated in TPO-stimulated platelets. Using a beta -casein promoter STATE binding site as a probe, we have also demonstrated that CrkL (a close relative of Crk) antiserum, but not Crk antiserum, supershifted the STAT5-DNA complex by an electrophoretic mobility shift assay, suggesting that CrkL, but not Crk, is the major component of the complex, Thus, Crk and CrkL may have distinct roles in the regulation of STATE.