Immediately following T cell receptor (TcR) ligation, cytoplasmic protein-tyrosine kinases (PTKs) are activated, resulting in the phosphorylation of intracellular and transmembrane proteins. p56[lck] and p59[fyn] are two Src family PTKs activated by TcR ligation. Following activation, Fyn interacts with several tyrosine-phosphorylated proteins, including SKAP55 (Src kinase-associated phosphoprotein of 55 kDa). SKAP55, which selectively binds Src kinase SH2 domains, contains a pleckstrin homology (PH) domain, a C-terminal Src homology 3 (SH3) domain, and multiple tyrosine phosphorylation sites. In addition, SKAP55 directly binds to FYB/SLAP-130, a novel substrate of TcR-stimulated protein tyrosine kinases. Like SLAP-130, SKAP55 is expressed exclusively in mononuclear cells, with preferential expression in T lymphocytes where it is constitutively phosphorylated in resting cells. A SKAP55 homolog, SKAP-HOM, has been identified which also interacts with SLAP-130, exhibits ubiquitous expression, and is phosphorylated only following T cell activation. Therefore, the Fyn-associated protein, SKAP55, along with SLAP-130, are components of a Fyn-mediated signaling cascade in T cells.