What guides migrating nerve axons to their correct targets during development? This is one of the basic issues in developmental neurobiology, and chemoattraction has long been thought to be involved. A tangible answer began to emerge with the demonstration that explants of rat spinal floorplate promoted the outgrowth of axons from commissural neurons, and caused these axons to turn. Subsequently, the specific factors concerned in axon turning were identified. These factors, called netrin-1 and netrin-2, are phlogenetically conserved molecules related to the product of the unc-6
gene in Caenorhabditis elegans.