Unlike PDE, PNPO deficiency lacks a specific biochemical marker in body fluids, but can be suspected on the basis of assays in blood and urine suggestive of l-aromatic acid decarboxylase deficiency (elevations in glycine, threonine, taurine, histidine, and low arginine) and treatment resistance to pyridoxine [23]. Here, PNPO is linked to hyperinsulinemic hypoglycemia, familial, 4.