In our set of 46 IEMs tested, four diagnoses remained undetected by the standard NGMS workflow: guanidinoacetate methyltransferase (GAMT) deficiency, argininosuccinic aciduria, dimethylglycine dehydrogenase deficiency, and lysinuric protein intolerance. The gene discussed is GAMT; the disease is hyperinsulinemic hypoglycemia, familial, 4.