Mutations in four genes with direct roles in axonal transport, DCTN1, TUBA4A, KIF1A and KIF5A, have been identified in familial forms of amyotrophic lateral sclerosis (DCTN1, TUBA4A, KIF5A) and hereditary spastic paraplegia (KIF1A and KIF5A) – with further mutations in DYNC1H1 (coding for the dynein heavy chain) linked to spinal muscular atrophy and Charcot-Marie-Tooth disease [30–35]. Here, KIF5A is linked to proximal spinal muscular atrophy.