The graphic shows half of a cross-section through human (a) and rodent (b) forebrains. Rakic and colleagues1 provide evidence that, during development, about 65% of interneurons in the human neocortex originate locally in the ventricular zone/subventricular zone (VZ/SVZ) of the neocortex. In rodents, by contrast, the main source of interneurons seems to be long-distance tangential migration of cells from the VZ of a subcortical area known as the ganglionic eminence. The other 35% of human interneurons seem to be produced by this route. In humans, interneurons originating from the cortical VZ/SVZ express both the transcription factors Mash1 and Dlx1/2 (cells with red nuclei); interneurons from the ganglionic eminence express only Mash1 (white nuclei).