Interest in glypican-3 (GPC3), a member of the glypican-related integral membrane heparan sulfate proteoglycans (GRIPS) family, has increased with the finding that it is mutated in the Simpson-Golabi-Behmel overgrowth syndrome (Pilia et al. [1996] Nat. Genet. 12:241-247). The working model suggested that the membrane-bound protein acts locally to limit tissue and organ growth and that it may function by interacting with insulin-like growth factor 2 (IGF2) to limit its local effective level. Here we have tested two predictions of the model. In situ hybridization with the mouse gene cDNA was used to study the expression pattern during embryonic and fetal development. In agreement with predictions, the gene is expressed in precisely the organs that overgrow in its absence; and the patterns of expression of Gpc3 and those reported for Igf2 are strictly correlated.