The identification of ALDH18A1 as new HSP-disease gene (SPG9) (5, 6) pointed toward the identification of a common biochemical pathway where two other well-known IEM-disease-genes (SLC25A15 in HHH syndrome and ARG1 in Argininemia) cause syndromes where spastic paraplegia is present and highly penetrant. The gene discussed is ARG1; the disease is ornithine translocase deficiency.