Nonketotic hyperglycinemia (NKH) is an inborn error of the glycine metabolism. A 9-year-old boy with learning disability and intermittent choreoathetosis during febrile illnesses had elevated plasma glycine level and CSF/plasma glycine ratio (0.044) and a novel homozygous missense mutation (c.605C>T; p.Ala202Val) in the GLDC gene, confirming the diagnosis of NKH. This is the first report of late-onset NKH with a confirmed underlying genetic defect. NKH should be in the differential diagnosis of intermittent choreoathetosis.
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