Glypicans are glycosylphosphatidylinositol-anchored heparan sulphate proteoglycans, which are thought to modulate signalling by growth factors, including fibroblast growth factors and Wnts. Studies in Xenopus have implicated glypicans, in particular glypican-4, in the process of convergent extension, which is essential for neural tube closure in Xenopus and mouse. Expression of glypican-4 has been reported in the mouse brain at embryonic day 10 and later stages, whereas expression during the developmental stages encompassing initiation and progression of neural tube closure has not been reported. Analysis by in situ hybridization reveals a complex pattern of glypican-4 mRNA localization at embryonic days 7-10.5, including sites in the anterior forebrain neuroepithelium, branchial arches, optic and otic vesicles, limb buds and somites. Glypican-4 expression is not detected in the midline of the embryo at the stage of initiation of neural tube closure, suggesting that glypican-4 is unlikely to play an essential role in convergent extension in the mouse.