WW domains are protein modules that mediate protein-protein interactions through recognition of proline-rich peptide motifs and phosphorylated serine/threonine-proline sites.WW domains are found in many different structural and signaling proteins that are involved in a variety of cellular processes.WW domain-containing proteins(WWCPs)and complexes have been implicated in major human diseases including cancer as well as in major signaling cascades such as the Hippo tumor suppressor pathway,making them targets for new diagnostics and therapeutics.