Development of sensitive methods to monitor and quantitatively assess the expression levels of endogenous genes and the association-interaction of proteins in living cells and whole organisms is a complex and challenging problem. In this chapter, we have described basic methods for investigating protein-protein interactions which include immunoprecipitation, GST pull-down assays, peptide bead pull-down assays, chemical crosslinking and photoaffinity labeling. These methods should provide important tools to dissect crosstalk between proteins and the direct implications of this crosstalk in signaling pathways and cancer biology.
