Hyperornithinemia-Hyperammonemia-Homocitrullinuria (HHH) syndrome, also known as ornithine translocase deficiency, is an inherited disorder that causes accumulation of toxic levels of ammonia in the blood. The severity and age of onset of the HHH syndrome varies widely among individuals, with onset in infancy having the most severe outcome. For most affected individuals, symptoms begin anywhere from around three years of age on into adulthood. Symptoms include episodes of low energy, vomiting, seizures, problems with coordination, confusion, or blurred vision. If these episodes occur frequently and are left untreated, individuals with HHH syndrome can develop learning disabilities, developmental delay, and stiffness caused by abnormal tensing of the muscles. People with HHH syndrome cannot tolerate high-protein foods, such as meat. Symptoms of HHH syndrome can be triggered by high-protein meals or stress caused by illness or periods without food. Healthy growth and development is possible by starting infants on a strict diet with supplements and medications, along with life-long medical surveillance. HHH syndrome is caused by pathogenic variants in the SLC25A15 gene.