Classic citrullinemia (CTLN1) is a disorder of the urea cycle characterized by neonatal or intermittent onset of hyperammonemia caused by a deficiency of the enzyme argininosuccinate synthetase. An acute neonatal form is the most common. Infants are normal at birth followed by an acute illness characterized by hyperammonemia, anorexia, vomiting, lethargy, and hepatomegaly in some cases, increased intracranial pressure, seizures and coma. A later onset presentation is less frequent and may be similar to or milder than the neonatal form and present with feeding difficulties, vomiting, episodic hyperammonemia, lethargy, seizures and cerebral atrophy on CT scans or MRI. CTLN1 has been diagnosed in adult women who developed severe symptoms such as psychosis during pregnancy or postpartum. Another late onset form, common in Japan, has been reported in individuals as late as 20 years of age with milder presentation including slurred speech, irritability and insomnia as presenting signs. Asymptomatic individuals have also been described.