Clinical, neurophysiological and neuroradiological findings in HHH syndrome resemble those observed in argininemia and pyrroline-5-carboxylate synthetase (P5CS) deficiency [33–36], two other disorders of aminoacid metabolism connected to the distal part of the urea cycle (Fig. 4). This evidence concerns the gene ALDH18A1 and ornithine translocase deficiency.