The Src family of protein tyrosine kinases, which includes Src, Lyn, Fyn, Yes, Lck, Blk, and Hck, are important in the regulation of growth and differentiation of eukaryotic cells . Src activity is regulated by tyrosine phosphorylation at two sites, but with opposing effects. While phosphorylation at Tyr416 in the activation loop of the kinase domain upregulates enzyme activity, phosphorylation at Tyr527 in the carboxy-terminal tail by Csk renders the enzyme less active .Fyn is a 59 kDa member of the Src family of tyrosine kinases. The carboxy terminus of Fyn shares extensive amino acid sequence homology with Src, but is very different within the amino-terminal 81 amino acid residues. The Fyn protein is synthesized and N-myristoylated on cytosolic polysomes and then rapidly targeted to the plasma membrane, where it is palmitoylated . The corresponding sequences surrounding Tyr416 and Tyr527 of Src are conserved in Fyn and thus may be similarly regulated by phosphorylation. Dually acetylated Fyn clusters in caveolae-like membrane microdomains and can interact with a variety of other signaling molecules. Fyn's biological functions are diverse and include signaling via the T cell receptor, regulation of brain function and adhesion mediated signaling . Alteration of the levels of Fyn in appropriate target tissues may lead to better treatments for some related diseases.