Roundabout (Robo) is the gene for the Roundabout (Robo) receptor. There is a family of Robo transmembrane receptors and they are the canonical receptors for the signaling molecule Slit, although there may be more Slit receptors besides Robo. Slit is a midline repellent molecule and Robo proteins are expressed on axons that avoid the midline, such as ipsilateral longitudinal axons in flies and post-crossing axons in flies and rodents. This Slit/Robo signaling is important in determining the trajectory of pioneer axons in the central nervous system. Together, they prevent longitudinal axons from crossing the midline of the brain and spinal cord.[1] They also prevent commissural axons from recrossing the midline. It is present in many animal species, including C. elegans nematode worms, Drosophila flies, chickens, rodents (including mice) and probably humans. There are several versions of this gene and they are referred to as Robo 1 and Robo 2 in the literature. They are thought to be largely genetically redundant but axon trajectories can be strongly disrupted when both are mutated.[2] In Drosophila, loss of multiple Robo proteins results in the collapse of longitudinal axons onto the midline. This is similar to Slit mutants.