ALDH4A1 and hyperprolinemia: Increased prolinemia (hyperprolinemia) was discovered as the cause of a metabolic syndrome in humans more than half a century ago, [7] and was classified as being type I or II depending on which enzyme is responsible for the deficiency (proline dehydrogenase causes type I hyperprolinemia, while P5C dehydrogenase causes type II hyperprolinemia) (reviewed by [8]).