Members of the ROBO family are a group of highly conserved transmembrane glycoproteins that make up a small subgroup of the immunoglobulin (Ig) superfamily. They are characterized structurally by five C2-type Ig-like repeats, three fibronectin type III domains, a transmembrane region, and an intracellular domain with three (ROBO3) or four (ROBO1, 2) CC (conserved cytoplasmic) motifs. ROBO4, a fourth putative family member, has been identified, but lacks one of the three fibronectin-like domains and two of the five Ig-like domains. ROBOs are best known for their roles as receptors for the Slit family of repellent axon guidance cues.