Proper neuronal migration is crucial for vertebrate nervous system development, but how do neurons know when to stop migrating? Hitoshi Okamoto and colleagues now shed light on this question and report that some neuronal progenitors in zebrafish fail to stop migrating at their normal position when a sugar called fucose is not synthesized correctly (see p. 1653). By screening for mutants in which vagus motor nuclei do not form properly, the authors isolated the towhead mutant and found that towheadencodes GDP-mannose 4,6, dehydratase (GMDS), a key enzyme in the fucosylation pathway. Accordingly, the authors detected fewer fucosylated glycans than normal in towhead mutant embryos, but although fucosylation has been reported to regulate Notch signalling, this signalling pathway is not altered in towhead mutants. The authors also demonstrate that, for correct migration, GMDS is not required in vagus motor neuron progenitors, but instead in the surrounding epithelial cells. They propose, therefore, that fucosylated glycans on epithelial cells prevent migrating vagus motor neuron progenitors from overshooting their target.