Carboxy-terminal Src kinase (Csk) is a ubiquitously expressed nonreceptor tyrosine kinase that negatively regulates the Src family kinases (SFK) by phosphorylation of the SFK carboxy-terminal tyrosine (1,2). Phosphorylated carboxy-terminal tyrosine binds to the SH2 domain of SFK intramolecularly and leads to folding and inactivation of the SFK (3). This Csk-catalyzed SFK tyrosine phosphorylation is highly specific and exclusive. The SFK carboxy-terminal tyrosine is the only known physiological substrate of Csk (4). Csk consists of an SH2, an SH3, and a kinase domain. There is evidence that the SH2 and SH3 domains are essential for the regulation of SFK, and Csk can be recruited to the membrane where SFKs are in an active state. This process is mediated by a Csk-binding protein (Cbp, also called PAG), which binds tightly to the SH2 domain of Csk (5). Activation of SFK by extracellular stimuli leads to the tyrosine phosphorylation of Cbp, generating docking sites for Csk. The recruitment of Csk forms a feedback mechanism for termination of SFK function (6).