A putatively hypermorphic de novo T99M variant, in the highly conserved P loop of the ATP binding site of the motor domain of KIF1A has been identified in a patient with ID (Hamdan et al., 2011), spasticity and axial hypotonia and in a second patient with a more severe phenotype including optic atrophy, growth failure and progressive cerebellar atrophy, in addition to ID (Citterio et al., 2015). Here, KIF1A is linked to Leber hereditary optic neuropathy.