Citrullinemia is a urea cycle disorder characterized by accumulation of ammonia and other toxic nitrogenous substances in the body. It is caused by defects in the ASS1 gene, which encodes argininosuccinate synthetase enzyme. Type I citrullinemia is the classic form of the disease that can be further classified based on its severity. The severe form, which is more common, has an early onset and the symptoms displayed by those affected include severe lethargy, poor feeding, seizures, and loss of consciousness. The milder form has its onset in childhood or early adulthood and is characterized by intense headaches, lethargy, ataxia, and balance and vision problems. Classic citrullinemia is estimated to affect about 1 in every 57,000 live births worldwide.