The Src family of protein tyrosine kinases (Src-PTKs) is important in the regulation of growth and differentiation of eukaryotic cells. The activity of Src-PTKs in cells of different types is negatively controlled by Csk. Csk binding protein (Cbp), also designated phosphoprotein associated with glycosphingo-lipid-enriched microdomains (GEMs) or PAG, is a ubiquitously expressed transmembrane phosphoprotein that binds specifically to the SH2 domain of Csk. Cbp is involved in the membrane localiz