Fyb (Fyn binding protein) and the anchoring proteins SKAP55 (src kinase-associated phosphoprotein) and SKAP55-R (SKAP55-related protein) associate with the tyrosine kinase p59fyn (1–3). SKAP55 and SKAP55-R bind to Fyb through their SH3 domains and function as substrates for p59Fyn in resting T cells (1–3). SKAP55 contains an amino-terminal pleckstrin homology domain and a carboxy-terminal SH3 domain binding motif of adjacent arginine and lysine residues followed by tandem tyrosines (i.e. RKxxYxxY) (4,5). SKAP55-R, similar in overall structure to SKAP55, contains a coiled-coil N-terminal domain (1,2). SKAP55 associates with SLAP-130, another component of the Fyn complex, which plays a role in the regulation of signaling events initiated by lymphocyte antigen receptors leading up to T cell activation (6). The human Fyb gene maps to chromosome 5p13.1 and encodes a 783 amino acid protein (7).