Protein phosphorylation on serine, threonine or tyrosine residues, can regulate protein-protein interactions via specific binding to the phospho-residues. In the February 21 Science Elia et al. describe a proteomic screen designed to isolate novel phospho-binding proteins (Science 2003, 299:1228-1231). The technique involves the creation of a library of biased partially degenerate phosphopeptides that are immobilized and used as a 'bait' in a screen for binding proteins. Elia et al. tested the technique with peptides resembling substrates of cyclin-dependent kinase and isolated the mitotic kinase Plk1 (polo-like kinase 1). The phospho-binding domain of Plk1 is important for localisation to the centrosome during mitosis. Elia et al. identified the phosphopeptide-binding domain and tested binding specificities, providing important proof-of-principle for their approach.