RA-GEF-1 (Rapgef2) is essential for proper development of the midline commissures

► Dorsal-telencephalon-specific RA-GEF-1 (cKO) mice develop commissural fiber defect. ► RA-GEF-1 is ubiquitously expressed in neurons but not in astrocytes. ► Probst bundles are formed at rostral levels of the cKO mice cerebral cortex. ► In cKO mice, indusium griseum glia and glial sling are absent at the cotical midline.