Stimulated by the recent success in determining the 3D (three-dimensional) structure of the mitochondrial uncoupling protein 2 by NMR [12], the present study was initiated in an attempt to reveal the action mechanism of HHH syndrome by investigating the structures of wild-type and mutated mitochondrial ornithine transporter-1. The gene discussed is SLC25A15; the disease is Hyperornithinemia-hyperammonemia-homocitrullinuria.