Crk was originally identified a part of fusion protein isolated from a chicken tumor virus. The CT10 (chicken tumor virus 10) retrovirus containing the viral crk (v-crk) oncogene was found to activate cellular tyrosine kinase activity in chicken embryo fibroblasts, thus the viral oncogene was named CT10 regulator of kinase (Crk). Isolation of v-crk led to the discovery of the cellular versions of Crk in chicken, human, mouse, and other eukaryotes. Crk possesses one SH2 domain and one SH3 domain. A second isoform of Crk (CrkII) has an additional SH3 domain. Crk and CrkII function as adaptor proteins involved in cell signaling, and they associate with a multitude of signaling proteins via their SH2 and SH3 domains. Although similar in structure and function, the biological activities mediated by the two isoforms (CrkI and CrkII) are reported to differ.