Previously, we had documented the presence of 4‐guanidinobutyrate in tissue and physiological fluids of both aldh5a1−/− mice and patients with SSADHD.39, 40 This species is predicted to derive from conjugation of GABA (in lieu of glycine) in the arginine:glycine amidinotransferase reaction of the creatine biosynthetic pathway. The gene discussed is GATM; the disease is succinic semialdehyde dehydrogenase deficiency.