C-terminal Src kinase (Csk) is a non-receptor protein tyrosine kinase which resembles Src-family kinases, but unlike them it lacks the conserved autophosphorylation site, the regulatory C-terminal tyrosine as well as myristylation and palmitylation. Csk negatively regulates Src-family kinases by phosphorylation of their C-terminal regulatory tyrosine. Disruption of the csk gene causes constitutive activation of Src-family kinases, and overexpression of Csk usually counteracts their signaling. The Csk-mediated regulation of those Src-family kinases, that are localized in lipid rafts, is enabled by a ubiquitously expressed transmembrane adaptor PAG (also known as Cbp, Csk-binding protein), which recruits Csk.