Citrullinemia type I is an inherited disorder that causes ammonia and other toxic substances to accumulate in the blood. This is due to a deficiency of the enzyme arginine succinate synthase. In most cases, the condition becomes evident in the first few days of life. Affected infants typically appear normal at birth, but soon afterwards experience a progressive lack of energy, poor feeding, vomiting, seizures, and loss of consciousness as ammonia builds up in their body. These medical problems are life-threatening in many cases but with appropriate treatment and dietary management, these infants survive an indeterminate period of time, sometimes without significant intellectual disability. A less common, milder form of type I citrullinemia can develop later in childhood or adulthood. Symptoms include intense headaches, partial loss of vision, problems with balance and muscle coordination, and lack of energy. Some individuals with citrullinemia type I never experience symptoms and some women may not have any medical issues until an onset of severe symptoms during pregnancy or postpartum. Citrullinemia type I is caused by pathogenic variants in the ASS1 gene.