During Drosophila development, some neuroepithelial cells become neuroblasts (neural stem cells) and generate the neuronal and glial cells of the fly's nervous system. But what controls the transition from neuroepithelial cell to neuroblast? In the Drosophila optic lobe,report Yasugi and colleagues, a wave of proneural gene expression that is negatively regulated by JAK/STAT signalling triggers neuroblast formation (see p. 1471). During optic lobe development, neuroepithelial cells at the medial edge of the outer optic anlage develop into neuroblasts, which generate the medulla neurons; those at the lateral edge develop into lamina neuron precursors. The researchers show that a wave of expression of the proneural protein Lethal of scute sweeps from the anlage's medial edge to its lateral edge and induces neuroblast formation. JAK/STAT signalling in the most lateral neuroepithelial cells negatively regulates this wave, they report. Thus, JAK/STAT signalling balances neuroblast and lamina neuron numbers in the optic lobe and allows the formation of a precise topographic map in the visual centre.