As a prototypical example, we focus here on the polo-box domain (PBD) of polo-like kinase 1 (Plk1), a serine/threonine kinase that is overexpressed in a wide range of cancers,7, 8 and is a known anticancer target due to its critical role in mitotic progression.9 The PBD helps in subcellular localization of the protein by binding to serine- or threonine-phosphorylated sequences at a polar phosphopeptide binding site (Figure 1 a).10, 11. This evidence concerns the gene PLK1 and cancer.