In the developing mouse brain, the balance between the self-renewal of neural precursor cells (NPCs) and their differentiation into neuronal and glial cells determines the final make-up of the brain. On p. 2791, Hirabayashi and co-workers propose that the Wnt signalling pathway directs neuronal differentiation in the developing mouse neocortex. They show that overexpression of Wnt7a or activated β-catenin inhibits NPC self-renewal and induces NPC differentiation in vitro and in vivo through the regulation of the proneural transcription factor neurogenin 1. Hirabayashi et al. reconcile these results with previous studies in which Wnt signalling promoted NPC self-renewal by showing that activated β-catenin only induces the differentiation of NPCs taken from 11.5-day or older embryos and not of those from younger embryos. They conclude that, as with wing disc development in Drosophila, Wnt signalling has stage-specific effects during vertebrate neural development.