Protein tyrosine phosphorylation has been the subject of intense investigation for over two decades. Tyrosine phosphorylation is catalyzed by members of the tyrosine kinase superfamily. To date, approximately 90 members of this superfamily have been identified in the human genome. Levels of phosphorylation of tyrosine residues within cellular proteins are observed to increase in response to growth factors, cytokines, insulin, extracellular matrix components and other stimuli. Tyrosine phosphorylation has several effects, including the regulation of enzyme activity, as well as the localization of enzyme activity through the generation of docking sites for proteins bearing SH2 domains.