Peripheral hyperprolinemia, which reflects CNS proline elevation,4, 5, 6, 7, 8, 9 has been associated with psychiatric disorders including schizophrenia.10, 11, 12 The proline dehydrogenase gene (PRODH) encodes proline oxidase (POX), the enzyme that catalyzes the first step in proline catabolism. Here, PRODH is linked to hyperprolinemia.