Certain KIF1A mutations, such as the Switch II and ATP-binding cassette mutants, E253K and T99M, cause a somal/proximal axon localization of cargos and motors which, taken together, may indicate that mislocalization of NGF underlies the Wallerian-like, or “dying-back” phenomena, seen in HSP. This evidence concerns the gene NGF and hereditary spastic paraplegia.