KIF1A-MD-T258M reduced the peripheral accumulation of WT KIF1A-MD effectively, although to a lesser extent than that exerted by KIF1A-MD-E253K carrying de novo mutation identified in global developmental delay with severe intellectual disability and to a great extent than the KIF1A-MD-A255V with a mutation from autosomal recessive HSP. The gene discussed is KIF1A; the disease is Global developmental delay.