The size, shape and structural integrity of the nervous system (and of other organs) depend on how many times its stem cells divide before differentiating. On p. 5539, Hatakeyama et al. report that the Hes genes, effectors of the Notch signalling pathway,control the timing of neural stem cell differentiation in the mouse nervous system. In normal mice, both neuroepithelial cells and the radial glial cells that are derived from them are neural stem cells; that is, they initially self-renew before undergoing terminal differentiation. In the absence of Hes1,Hes3 and Hes5, the researchers show that neuroepithelial cells form normally but are not properly maintained, and that radial glial cells prematurely differentiate into neurons. The resultant disorganisation of the nervous system in these mice, suggest the researchers, is because the missing radial glial cells normally form the structures that maintain neural tube morphology.