The developing central nervous system is divided into compartments (which later form distinct populations of neurons) by boundary cells. These slowly proliferating, non-neuronal cells control neuronal specification in neighbouring compartments, but what regulates boundary formation? Baek and colleagues now report that persistent high levels of the bHLH transcription factor Hes1 regulate the formation of five boundaries (the floor plate,isthmus, rhombomere, roof plate and zona limitans intrathalamica boundaries)in the developing mouse brain (see p. 2467). They show first that Hes1 expression in non-boundary cells is variable but is always high in boundary cells. Then, by manipulating the expression of Hes1 in both cell types in vivo and in vitro, the researchers demonstrate that persistent high levels of Hes1 expression repress the expression of proneural transcription factors and reduce cell proliferation rates. Conversely, the absence of Hes1 (and also of Hes3 and Hes5)disrupts the organizing centres of the developing nervous system, thus confirming the importance of Hes1 expression in boundary formation.