Transcription factor required for ciliogenesis and islet cell differentiation during endocrine pancreas development. Essential for the differentiation of nodal monocilia and left-right asymmetry specification during embryogenesis. Required for the biogenesis of motile cilia by governing growth and beating efficiency of motile cells (PubMed:15121860, PubMed:19671664). Also required for ciliated ependymal cell differentiation (PubMed:16930429). Together with RFX6, participates in the differentiation of 4 of the 5 islet cell types during endocrine pancreas development, with the exception of pancreatic PP (polypeptide-producing) cells (PubMed:17229940). Regulates transcription by forming a heterodimer with another RFX protein and binding to the X-box in the promoter of target genes (By similarity). Regulates the expression of genes involved in ciliary assembly (DYNC2LI1, FOXJ1 and BBS4) and genes involved in ciliary motility (DNAH11, DNAH9 and DNAH5). Represses transcription of MAP1A in non-neuronal cells but not in neuronal cells. Heterodimer; heterodimerizes with RFX1 and RFX2, and RFX6. Expressed in ciliated cells of the node and in the ciliated ependymal cells of the subcommissural organ (SCO), choroid plexuses (CP) and ventricular walls during embryonic and postnatal development. Expressed in developing and mature pancreatic endocrine cells during embryogenesis and in adults (at protein level). High rate of embryonic lethality. 2 peaks of death are observed: approximately half of the embryos die around days 11 or 12 of embryonic development. Of the embryos that survive past this stage, approximately two-thirds die at birth. Surviving mice are systematically smaller. Their body weights at birth are approximately one-third lower. This growth retardation increases with age, and the body weights that adult male or female mice attain are less than half those of wild-type mice. Mice exhibit a pronounced defect in nodal cilia: cilia are present but remain markedly stunted. Mice also suffer from hydrocephalus without stenosis of the aqueduct of Sylvius. In pancreatic endocrine cells, primary cilia are reduced in number and severely stunted: this ciliary abnormality is associated with a developmental defect leading to an altered cellular composition of the islets of Langerhans. Just before birth, islets contain considerably less insulin-, glucagon-, and ghrelin-producing cells, whereas pancreatic PP (polypeptide-producing) cells are markedly increased in number. In adult mice, the defect leads to small and disorganized islets, reduced insulin production, and impaired glucose tolerance. Belongs to the RFX family.