PNPO and hyperprolinemia type 2: PLP depletion is the main pathophysiological mechanism for different inborn errors of metabolism, including ALDH7A1 deficiency, PNPO deficiency, PLP binding protein deficiency, hyperprolinemia type II, hypophosphatasia, and glycosylphosphatidylinositol (GPI) anchor synthesis defects (Table 1) [3].