v-Crk is a member of the family of adaptor-type signaling molecules that consist mostly of SH2 and SH3 domains. The cellular homologs of v-Crk includes CrkI, CrkII, and CrkL; these have been isolated from species ranging from lower vertebrates to man. Crk-family proteins are involved in a variety of signaling cascades such as those of growth factor receptor, integrin, T-cell receptor, B-cell antigen receptor, and cytokines. It has been postulated that the primary function of Crk is to recruit cytoplasmic proteins in the vicinity of tyrosine kinases through SH2-phosphotyrosine interaction. Thus, the output from Crk depends on the SH3-binding proteins, which include the C3G guanine nucleotide exchange protein for Rap1, Abl tyrosine kinase, DOCK180, and the Sos guanine nucleotide exchange protein for Ras. The variety of the Crk-binding proteins indicate the pleiotropic function of Crk.