Definition:

Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is a rare autosomal recessive disorder, characterized by mental retardation, progressive spastic paraparesis, seizures, and myoclonus epilepsy. This disease varies widely in its severity and age of onset. The HHH syndrome is thought to be caused by the defective activities of the mitochondrial carrier responsible for transporting ornithine from the cytoplasm into the inner mitochondrial membrane. Mutations in the SLC25A15 gene, that encodes the mitochondrial ornithine transporter have been shown to be correlated with the HHH syndrome.