The construction of the anatomically distinct processing centres of the brain requires the strict segregation of adjacent groups of cells during morphogenesis. On p. 5935, Tayler and co-workers report that Slit and the Robo proteins, molecules that are better known for their role in axon guidance, control the compartmentalisation of visual centres during Drosophila brain development. They show that the secreted protein Slit surrounds the lamina glia and that the three fly Robo proteins (receptors for Slit) are expressed in distal cell neurons in the lobula cortex. Loss of Slit expression or inhibition of Robo causes the lamina glia and distal cell neurons to intermingle. Thus, Slit keeps Robo-expressing neurons within the confines of the lobula cortex and establishes a sharp boundary between the lamina and lobula. In another paper,Bénard and colleagues describe MAU-2, a novel Caenorhabditis elegans axon guidance factor (see p. 5947). Whether MAU-2,like Slit and Robo, has additional functions remains to be seen.