Protein–protein interactions mediated by proline-rich motifs are involved in regulation of many important signaling cascades. Protein domains specialized in recognition of these motifs expose a flat and relatively rigid binding site that preferentially interacts with sequences adopting a left-handed polyproline helix II. Here, we present a toolkit of new chemical entities that enables rational construction of selective small-molecule inhibitors for these protein domains. As proof of principle, we developed a selective, cell-permeable inhibitor of