The integration of multiple transmembrane signals is especially important during development and maintenance of the nervous system, communication between cells of the immune system, evolution of transformed cells, and metabolic control (Hunter 1997). Tyrosine phosphorylation plays a key role in many of these processes by directly controlling the activity of receptors or enzymes at early steps in signaling cascades, or by the assembly of multicomponent signaling complexes around activated receptors or their cellular substrates (Pawson 1995). In most if not all cases, initialization of the signaling cascade controlled by growth factor and cytokine receptors originates with multisite tyrosine phosphorylation catalyzed directly by kinases activated during ligand-induced dimerization of specific membrane receptors (Schlessinger 1988; Helding 1995). In many cases, tyrosine autophosphorylation sites in activated receptors directly bind signaling proteins containing Src homology-2 domains (SH2 proteins). In other cases, tyrosine autophosphorylation increases the activity of the receptor kinase, which mediates tyrosine phosphorylation of cytosolic substrates or docking proteins that recruit SH2 proteins into multipotential signaling complexes (Myers and White 1995). The network is further elaborated through other modules which mediate protein-protein or protein-lipid interactions, including PTB, PDZ, SH3, WW, and PH domains.