Three tyrosine residues at positions 771, 783 and 1254 in PLC-gamma1 have been identified as the sites of receptor tyrosine kinase phosphorylation. Of these Y783 and Y1254 are required for activation of PLC-gamma1. The phosphorylation of the tyrosine residues and the activation of PLC-gamma1 is mediated by both Syk tyrosine kinase ZAP-70 and Tec kinase ITK. Immunoglobulin superfamily member 2 (IgSF2 or cell surface glycoprotein V7)ligation interferes with T cell activation and IL-2 secretion through a Ca2+ and tyrosine kinase-dependent pathway that inhibits PLC-gamma1 phosphorylation and prevents NF-AT translocation to the nucleus (Soares et al. 1998, 1997).