Csk (Carbo, Xenopus/Amphibian,y-terminal Src kinase) is a ubiquitously e, Xenopus/Amphibian,pressed nonreceptor tyrosine kinase. Csk negatively regulates the Src family kinases (SFK) by phosphorylation of the SFK carbo, Xenopus/Amphibian,y-terminal tyrosine. Phosphorylated carbo, Xenopus/Amphibian,y-terminal tyrosine binds to the SH2 domain of SFK intramolecularly and leads to folding and inactivation of the SFK. This Csk-catalyzed SFK tyrosine phosphorylation is highly specific and e, Xenopus/Amphibian,clusive. The SFK carbo, Xenopus/Amphibian,y-terminal tyrosine is the only known physiological substrate of Csk. It is e, Xenopus/Amphibian,pressed in lungs and macrophages.