The pathomechanisms behind PLP depletion include the accumulation of metabolites inactivating PLP (ALDH7A1 deficiency), the impairment of PLP synthesis and recycling (PNPO deficiency), defects in the intracellular transport of PLP (PLPBP deficiency), or impaired transport of PLP into the brain (hypophosphatasia and GPI anchor defects) [4]. This evidence concerns the gene PNPO and hyperinsulinemic hypoglycemia, familial, 4.