Spinal cord myelination crucially depends on the long-distance migration of oligodendrocyte precursors (OPs) that originate from a ventral ventricular zone (VVZ) domain in the developing neural tube. Tsai et al. now report that the initial dispersal of OPs from this location is mediated by a chemorepulsive response to netrin 1, which belongs to the well-known family of neuronal chemorepellants and chemoattractants. They show, on p. 2095, that netrin 1 is expressed in chick ventral spinal cord when OPs initially migrate from the VVZ. In vitro, both chick ventral spinal cord explants and netrin 1-positive cells repel migratory OPs, which express the netrin 1 receptors DCC and UNC5. This chemorepulsive response can be inhibited by a function-blocking, anti-DCC antibody. These, and other, findings reported here highlight the crucial role of netrin 1 in the process of axon myelination.