Proteus syndrome is a complex hamartomatous disorder characterized by asymmetrical gigantism, epidermal nevi, vascular malformations, hamartomas, lipomas, and hyperostosis. Since the syndrome was first described, many hypotheses have been proposed to explain its occurrence. The most plausible is Happle's somatic mosaic hypothesis, but no somatic mutations in candidate genes have been reported to be clearly involved in Proteus syndrome. However, germ-line PTEN mutations have been reported in patients with Proteus and in "Proteus-like disorders." Other studies of patients with Proteus syndrome have not supported these findings. In this study, affected and unaffected tissue from six patients diagnosed with Proteus syndrome were screened by direct sequencing of genomic DNA to determine if there might be an association between germ-line or somatic mutations in PTEN or GPC3 and the development of Proteus syndrome. No intra-exonic mutations were identified, indicating that neither PTEN nor GPC3 are likely to have major roles in the etiology of Proteus syndrome in our series of patients.