Spastin localises to centrosomes and spindle poles in dividing cells [39], and to the distal ends of axons, where it regulates axon branching [40]; mutations in the spastin gene, SPG4, account for 40% of cases of autosomal dominant Hereditary Spastic Paraplegia (HSP), which is characterised by the retrograde degeneration of corticospinal tracts (the longest axons), leading to progressive weakness and spastic paralysis of the lower limbs [41]. The gene discussed is SPAST; the disease is hereditary spastic paraplegia.