KIF21A and hereditary spastic paraplegia: Charcot-Marie-Tooth disease type 2A can be caused by mutations in Kif1b (Zhao et al., 2001), congenital fibrosis of the extraocular muscles can be caused by mutations in Kif21a, and a loss-of-function mutation in Kif5a motor domain can result in hereditary spastic paraplegia (Reid et al., 2002; Xia et al., 2003; Tessa et al., 2008; Morfini et al., 2009).