 The ratio of peak intensities in the mass spectrum for such peptide pairs reflects the abundance ratio for the two proteins.A SILAC approach involving incorporation of tyrosine labeled with nine carbon-13 atoms (13C) instead of the normal carbon-12 (12C) has been utilized to study tyrosine kinase substrates in signaling pathways.SILAC has emerged as a very powerful method to study cell signaling, post translation modifications such as phosphorylation, protein–protein interaction and regulation of gene expression.