The importance of inhibitory (as well as stimulatory) signals for cell adhesion and/or migration is most readily recognizable during development of the nervous system, where pathfinding and the avoidance of inappropriate tracks by axons in the periphery, and the development of organized layers and establishment of boundaries in the central nervous system are crucial events. Therefore, most of the recent progress concerning the identification of novel important extracellular molecules involved in repulsive and inhibitory interactions has been made by neurobiologists.