Citrullinemia type 1 (CTLN1) is an autosomal recessive urea cycle disorder caused by deficiency of the cytosolic enzyme argininosuccinate synthetase 1 (ASS1‐D; MIM #215700) due to pathogenic variants in the ASS1 gene located on chromosome 9q34.11. The gene discussed is ASS1; the disease is citrullinemia type I.