New roles for an old molecule in axon-glial interaction.

Axons need to be above a minimum size before they can be ensheathed by myelin-forming glia. But it has generally been assumed that the axonal signals that initiate myelination, whatever they are, would act similarly in both the CNS and the PNS. The surprising finding of Chan et al. in this issue of Neuron is that NGF can act as a regulator of ensheathment but that it has opposite effects on CNS and PNS axons.