Roundabout homolog 4 (ROBO4) is also known as Magic roundabout (MRB) and ECSM4, which belongs to the immunoglobulin superfamily or ROBO family. ROBO4 contains 2 fibronectin type-III domains and 2 Ig-like C2-type (immunoglobulin-like) domains. ROBO4 is specifically expressed in endothelial cells and is also expressed at sites of angiogenesis in different tumor types. ROBO4 / MRB is the receptor for Slit proteins, at least for SLIT2, and seems to be involved in angiogenesis and vascular patterning. ROBO-4 may mediate the inhibition of primary endothelial cell migration by Slit proteins.