Protein tyrosine phosphatases, or PTPs, are type I transmembrane proteins, membrane-associated proteins or proteins localized in nuclei. Examples of transmembrane PTPs are LAR, PTPa, PTPb, PTPg, PTPd, PTPe, PTPz, PTPk and PTPm. Transmembrane PTPs play diverse roles during development and in adult tissues. Immunodepletion studies have suggested LAR to be a regulator of insulin receptor phosphorylation. Trio is a LAR-interacting protein that contains two functional guanine nuclear exchange factor (GEF) domains and a serine/threonine protein kinase (PSK) domain. One of the the Trio-GEF domains exhibits Rac-specific GEF activity while the other exibits Rho-specific GEF activity. The carboxy terminal PSK domain is most similar to the PSK domains of the CaMK family.