Slit-Robo is the name of a cell signaling protein complex with many diverse functions including axon guidance and angiogenesis. Slit refers to a secreted protein that is most widely known as a repulsive axon guidance cue, and Robo refers to its transmembrane protein receptor. There are four different Robos and three Slits in vertebrates: Robo1, Robo2, Robo3/Rig-1, and Robo4, and Slit1, Slit2, Slit3. There are three Robos and a single Slit in Drosophila. The corresponding Slit and Robo homologues in C. elegans are Slt and Sax-3, respectively. Slits are characterized by four distinct domains, each containing variable numbers of leucine-rich repeats (LRRs), seven to nine EGF repeats, an ALPS domain (Agrin, Perlecan, Laminin, Slit), and a cysteine knot. Robos are characterized by five Ig-like domains, three fibronectin type III (FNIII) repeats, a transmembrane portion, and an intracellular tail with up to four conserved cytoplasmic motifs: CC0 (a potential site of tyrosine phosphorylation), CC1 (also a potential site of tyrosine phosphorylation and binds P3 domain of netrin-1 receptor DCC), CC2 (polyproline stretch; consensus binding site for Ena/Vasp proteins), and CC3 (polyproline stretch). Background and discovery In the developing nervous system of bilaterians, most axons cross over to the opposite (contralateral) side of the body. What are the genes that ensure that this process occurs appropriately? This fundamental question in axon guidance led researchers to Robo, which was identified in a large-scale screening of Drosophila mutants in the early 1990s. Robo expression was shown to be required for repulsion of axons from the midline, both in ipsilateral axons that never cross the midline and in commissural axons that had already crossed. Another protein Commissureless (Comm) was found to be an essential regulator of Robo: in comm mutants, Robo activity is too high, and no axons cross the midline. Several years later, genetic evidence, biochemical binding experiments, and explant assays identified Slits as the repulsive ligands for Robo receptors in both Drosophila and vertebrates. Slit was also found to act as a repulsive cue in olfactory bulb guidance. The high conservation of Slit and Robo structures and the similarities in their function among vertebrates and invertebrates make a strong case for an evolutionarily conserved requirement for Slit/Robo signaling in the developing nervous system. Cell signaling pathways Slit-robo binding The functional region of Slit proteins is located within the leucine-rich repeats (LRRs). Slit2 binds Robo1 in a flexible linkage between its D2 domain and