Engagement of the T cell receptor (TcR) results in the activation of multiple intracellular src protein tyrosine kinases (PTKs) including p56 [lck] and p59 [fyn]. PTK activation induces TcRζ phosphorylation and the subsequent recruitment and activation of ZAP-70 which specifically phosphorylates SLP-76. SLP-76 interacts with multiple components of T cell signaling pathways, including Grb-2, PLC-γ, Vav, and SLAP-130 (also known as SLP-76 Associated Phosphoprotein of 130 kDa). SLAP-130 contains multiple tyrosine based motifs, proline base type I and II SH3 domain binding motifs, lysine/glutamic acid rich nuclear localization motifs, and an SH3-like domain. It associates with the SH2 domain of SLP-76 and is phosphorylated by TcR induced PTKs. This protein has also been reported to directly interact with fyn and has been referred to as fyn binding protein (FYB). Co-transfection data has shown that a limited concentration of FYB/SLAP-130 enhances TcR-induced IL-2 production, while overexpression downregulates gene expression. Thus, FYB/SLAP-130 is viewed to be a regulatory component of the fyn/SLP-76 signaling cascade in T cells.