Abstract : Regulatory Factor X (RFX) transcription factors are conserved in wide range of species. We work on a murine model, that is inactivated for Rfx3. We show here that Rfx3 deficient mice suffer from numerous brain defects as the abnormal development of the Choroïd Plexuses, Sub-Commissural Organ, Pineal Gland and Posterior Commissure. And fine analysis, realized at early stages of embryonic development, reveals that these brain defects could be due to a mis-specification of the forebrain dorsal midline. As RFX3 is a ciliogenic regulator, these data support a new ciliogenic function in brain morphogenesis. Moreover, these mice suffer from accallosality, a classic ciliopathy phenotype. And a mis-localization of corticoseptal cells, involved in callosal guidance, could be explain this cerebral affection. Thus this work will bring new understandings of the physiopathological mechanisms of ciliopathies.