citrullinemia type I (Monarch Disease Ontology term MONDO_0008988) is a rare autosomal recessive urea cycle defect characterized biologically by hyperammonemia and clinically by progressive lethargy, poor feeding and vomiting in the neonatal form (Acute neonatal citrullinemia type I) and by variable hyperammonemia in the later-onset form (adult-onset citrullinemia type I). Also known as: ASS deficiency, CTLN1, argininosuccinate synthase deficiency, argininosuccinate synthetase deficiency, argininosuccinic acid synthase deficiency, argininosuccinic acid synthetase deficiency, citrullinemia type 1, classic citrullinemia.