AbstractSLC1A4 is a trimeric neutral amino acid transporter essential for shuttling L‐serine from astrocytes into neurons. Individuals with biallelic variants inSLC1A4are known to have spastic tetraplegia, thin corpus callosum, and progressive microcephaly (SPATCCM) syndrome, but individuals with heterozygous variants are not thought to have disease. We identify an 8‐year‐old patient with global developmental delay, spasticity, epilepsy, and microcephaly who has ade novoheterozygous three amino acid duplication inSLC1A4(L86_M88dup). We demonstrate that L86_M88dup causes a dominant‐negative N‐glycosylation defect of SLC1A4, which in turn reduces the plasma membrane localization of SLC1A4 and the transport rate of SLC1A4 for L‐serine.