Protein kinases can be classified according to whether they phosphorylate phenolic (tyrosine) or aliphatic (serine and threonine) hydroxyl groups (Hunter and Cooper, 1985). Although there are now a few examples of kinases that seemingly break the rule (Howell et al., 1990), most tyrosine kinases differ from the serine/threonine kinases in their primary amino acid sequences. Consequently, as open reading frames in cDNA clones have been sequenced it has been relatively easy to predict whether they might encode tyrosine kinases (Hanks et al., 1988). This has lead to a rapid expansion of the tyrosine kinase family.