nonreceptor tyrosine kinases u2014 have never before been linked to ppar activation or to diabetes obesity or other conditions related to metabolic syndrome nor has any other non receptor protein tyrosine kinase a brief background on c src is given here c src is a protein tyrosine kinase tyrosine kinases are enzymes that catalyze the transfer of the terminal phosphate of adenosine triphosphate to tyrosine residues in protein substrates tyrosine kinases are believed by way of substrate phosphorylation to play critical roles in signal transduction for a number of cell functions though the exact mechanisms of signal transduction is still unclear tyrosine kinases have been shown to be important contributing factors in cell proliferation carcinogenesis and cell differentiation tyrosine kinases can be categorized as receptor type or non receptor type receptor type tyrosine kinases have an extracellular a transmembrane and an intracellular portion while non receptor type tyrosine kinases are wholly intracellular the receptor type tyrosine kinases are comprised of a large number of transmembrane receptors with diverse biological activity in fact about twenty different subfamilies of receptor type tyrosine kinases have been identified one tyrosine kinase subfamily designated the her subfamily is comprised of egfr her2 her3 and her4 ligands of this subfamily of receptors include epithileal growth factor tgf alpha amphiregulin hb egf betacellulin and heregulin.