KIF21A and autosomal dominant childhood-onset proximal spinal muscular atrophy with contractures: Previous results have identified relationships between diseases and gain-of-function mutations in genes involved in axonal transport, such as SPG30 and KIF1A, congenital fibrosis of the extraocular muscle type 1 and KIF21A, and autosomal dominant lower extremity-predominant spinal muscular atrophy-2A and BICD2 [48–51].