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Nuclear factor I (NFI) genes are expressed in multiple organs throughout development (; for review, see). All four NFI genes are expressed in embryonic mouse brain, with Nfia, Nfib, andNfix being expressed highly in developing cortex . Disruption of the Nfia gene causes agenesis of the corpus callosum (ACC), hydrocephalus, and reduced GFAP expression . Three midline structures, the glial wedge, glia within the indusium griseum, and the glial sling are involved in development of the corpus callosum (; ; ). BecauseNfia^−/−mice show glial abnormalities and ACC, we asked whether defects in midline glial structures occur inNfia^{− / −} mice. NFI-A protein is expressed in all three midline populations. InNfia^{− / −} , mice sling cells are generated but migrate abnormally into the septum and do not form a sling. Glia within the indusium griseum and the glial wedge are greatly reduced or absent and consequently Slit2 …