Robo4 is a vascular specific receptor that is a repulsive cue to migrating endothelial cells during vascular development. Guidance and patterning of axons are orchestrated by cell surface receptors and ligands that provide directional cues. Interactions between the Robo receptor and Slit ligand families of proteins initiate signaling cascades that repel axonal outgrowth. In contrast to known neuronal Robo family members, the arrangement of the extracellular domains of Robo4 diverges significantly from that of all other Robo family members. Robo4 is specifically expressed in the vascular endothelium during murine embryonic development; the only Robo family member expressed in primary endothelial cells. Robo4 also has a role in regulating the migration of endothelial cells during tumor angiogenesis.