Simpson-Golabi-Behmel syndrome (SGBS) is an X-linked syndrome characterized by pre- and postnatal overgrowth (gigantism), which clinically resembles the autosomal Beckwith–Wiedemann syndrome (BWS). Deletions and translocations involving the glypican-3 gene (GPC3) have been shown to be associated with SGBS. Occasionally, these deletions also include the glypican-4 gene (GPC4). Glypicans are heparan sulfate proteoglycans which have a role in the control of cell growth and cell division. We have examined the mutational status of the GPC3 and GPC4 genes in one patient with Perlman syndrome, three patients with overgrowth without syndrome diagnosis, ten unrelated SGBS-patients and 11 BWS patients. We identified one SGBS patient with a deletion of a GPC3 exon. Six SGBS patients showed point mutations in GPC3. One frameshift, three nonsense, and one splice mutation predict a loss-of-function of the glypican-3 protein. One missense mutation, W296R, changes an amino acid that is conserved in all glypicans identified so far. A GPC3 protein that reproduces this mutation is poorly processed and fails to increase the cell surface expression of heparan sulfate, suggesting that this missense mutation is also a loss-of-function mutation. In three SGBS patients and in all non-SGBS patients, no mutations could be identified. We found three single nucleotide polymorphisms in the GPC4 gene but no evidence for loss-of-function mutations in GPC4associated with SGBS.