Mutations in the SPAST gene are a frequent cause of hereditary spastic paraplegias, which are incurable neurodegenerative disorders characterised by progressive weakness and spasticity of the legs. Using the zebrafish CNS as a model system, Butler et al. show that SPAST is a crucial regulator of nerve fibre outgrowth, through its role in controlling microtubule dynamics and growth-cone motility.

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