A short-range repellent, controlling axon crossing of the midline and a long-range chemorepellent, controlling mesoderm migration and patterning away from the midline. May interact with extracellular matrix molecules. Repulsive ligand for the guidance receptor roundabout (robo) and prevents inappropriate midline crossing by Robo-expressing axons. Interacts with robo. In embryos, highest expression occurs around the midline glia and low expression is observed around CNS axons lateral to the midline. Expression can be seen on the commissural axons traversing the glial cells but it is absent from the cell bodies of these neurons. Flies lacking sli exhibit disruption of the developing midline cells and the commissural axon pathways. Truncated N-terminus.