De novo KIF1A variants with AD transmission have been identified in multiple cases with childhood onset of intellectual disability and a number of neurological signs, such as progressive spastic paraplegia, optic nerve atrophy, peripheral neuropathy and cerebral and/or cerebellar atrophy, have been variously classified as autosomal dominant mental retardation type 9 (MRD9, OMIM#614255) [24–28] or complicated hereditary spastic paraplegia [25, 29, 30]. Here, KIF1A is linked to intellectual disability, autosomal dominant 9.