With an aim to provide a novel factor inducing proliferation of neural stem cells and differentiation of these cells into nerve cells, a pharmaceutical composition comprising 1) CRBN, 2) a nucleic acid encoding CRBN, or 3) a stem cell or a neural progenitor cell in which CRBN is expressed, a method including administering the pharmaceutical composition to a non-human animal and inducing proliferation of neural stem cells or neural progenitor cells of the non-human animal and differentiation of these cells into nerve cells, and a method for screening for a therapeutic drug for a disease of cerebral cortex or a surgical injury of cerebral cortex, using CRBN, are provided.