The name glypican has been assigned to a family of heparan sulfate (HS) proteoglycans that are linked to the cell membrane by a glycosyl-phosphatidylinositol anchor. To date, six family members of this family have been identified in mammals (GPC1 to GPC6) and two in Drosophila. Glypicans are expressed predominantly during development, and they are thought to play a role in morphogenesis. As HS-carrying molecules, glypicans were initially considered potential regulators of heparin-binding growth factors. This has been recently confirmed by genetic interaction experiments showing that glypicans regulate wingless signaling in Drosophila. The involvement of glypicans in the in vivo regulation of other heparin-binding growth factors, such as fibroblast growth factors, remains to be determined. Interestingly and unexpectedly, a role for GPC3 in the regulation of insulin-like growth factors has been proposed. This hypothesis is based on the phenotype of patients with Simpson-Golabi-Behmel syndrome (SGBS), an overgrowth and dysmorphic syndrome in which the GPC3 gene is mutated. Thus, it is possible that glypicans regulate different kinds of growth factors in a tissue-specific manner. In addition to its involvement in SGBS, down-regulation of GPC3 has been recently associated with the progression of several types of malignant tumors, including mesotheliomas and ovarian cancer. A role for GPC1 in pancreatic cancer progression has also been proposed.