The identification of mutations in ALDH18A1 causing the complicated HSP form SPG9 (P5CS deficiency, SPG9A MIM601162; SPG9B, MIM616586) [31,32] pointed toward the identification of a common biochemical pathway where two other well-known Inborn Errors of Metabolism, Hyperornithinemia, Hyperammonemia, and Homocitrullinuria syndrome (HHH, MIM 238970) and arginase deficiency (ARG1 deficiency, MIM 207800), are characterized by a phenotype where spastic paraplegia is present and highly penetrant [33,34,35]. The gene discussed is ALDH18A1; the disease is Hyperammonemia.