IGSF4 is a novel immunoglobulin (Ig)-like intercellular adhesion molecule. Since IGSF4 has been characterized by several independent research groups, this molecule is called by three names, TSLC1, SgIGSF and SynCAM. In the experiments to study global changes of gene expression in fetal murine brains after prenatal exposure to low-doses of X-rays, we have found IGSF4 as one of down-regulated genes after X-irradiation. In order to elucidate the expression of spatiotemporal expression of IGSF4 in the developing brain, we have produced polyclonal antibody against IGSF4 and studied the expression of IGSF4 with immunohistochemistry and Western blot analysis. At embryonic day (E) 12.5, IGSF4-immunoreactivity (IR) was observed diffusely in the telencephalic wall, whereas it became rather confined to the subplate, the cortical plate and the subventricular zone as the development proceeded. Noteworthy was a distinct radial pattern found in the cortical plate of E16.5. IGSF4-IR gradually decreased after birth and disappeared in adulthood. In the cerebellum, IGSF4 was expressed in the molecular layer at postnatal day (P) 0 through P14. By Western blot analysis, IGSF4 remained at low levels throughout embryonic stage, whereas it increased after birth. These spatiotemporal patterns of the expression suggest that IGSF4 plays crucial roles in the development of both telencephalon and cerebellum.