Glypican-3 (GPC3) belongs to a six-member family of membrane-bound heparan sulfate proteoglycans with a gene that is localized to Xq26.2. GPC3 belongs to a family of cell-surface heparan sulfate proteoglycans and is highly expressed during embryogenesis and organ development. GPC3 has limited expression in adult tissues, including lung, ovaries, mammary epithelium, and mesothelium. Normally, the liver does not express detectable levels of GPC3, but it has been observed to be highly upregulated in hepatocellular carcinomas and hepatoblastomas. Glypican-3 has been shown to play a role in the control of cell growth through the induction of apoptosis. Mutations in glypican-3 result in cell overgrowth. Among all mammalian glypicans, only mutation of GPC3 is associated with a phenotype in humans. GPC3 spans more than 500 kb of genomic DNA and contains 8 exons. Loss-of-function mutations in GPC3 result in overgrowth of many organs (Simpson-Golabi-Behmel syndrome), including the liver, which has been confirmed using GPC3-deficient mice. This observation suggests that GPC3 plays a role in growth suppression and organ size control. GPC3 is highly expressed in yolk sac tumors and choriocarcinomas, with a cytoplasmic staining pattern. It is only variably expressed in teratomas and is absent in germinomas and embryonic carcinomas.