AbstractThe Mrj co‐chaperone is expressed throughout the mouse conceptus, yet its requirement for placental development has prohibited a full understanding of its embryonic function. Here, we show that Mrj−/− embryos exhibit neural tube defects independent of the placenta phenotype, including exencephaly and thin‐walled neural tubes. Molecular analyses revealed fewer proliferating cells and a down‐regulation of early neural progenitor (Pax6, Olig2, Hes5) and neuronal (Nscl2, SCG10) cell markers in Mrj−/− neuroepithelial cells. Furthermore, Mrj−/− neurospheres are significantly smaller and form fewer secondary neurospheres indicating that Mrj is necessary for self‐renewal of neural stem cells. However, the molecular function of Mrj in this context remains elusive because Mrj does not colocalize with Bmi‐1, a self‐renewal protein. Furthermore, unlike in Mrj−/− placentas, intermediate filament‐containing aggregates do not accumulate in Mrj−/− neuroepithelium, ruling out nestin as a substrate for Mrj. Regardless, Mrj plays an important role in neural stem cell self‐renewal. Developmental Dynamics 238:2564–2574, 2009. © 2009 Wiley‐Liss, Inc.