Citrin deficiency is one of the most frequent diseases among congenital metabolic disorders in Japan. Based on the time of its onset, citrin deficiency is classified into neonatal intrahepatic cholestasis caused by citrin deficiency (NICCD; OMIM # 603471) and adult onset type II citrullinemia (CTLN2; OMIM # 605814). Citrin deficiency has been found to be caused mainly by mutations in the SLC25A13 gene, and the prevalence of homozygotes for SLC25A13 polymorphism is 1/17,000 in Japan . The prevalence of heterozygous carriers is 0.015 in Japan, which is higher than that in the western countries . Nevertheless, since the prevalence of CTLN2 is 1/100,000 to 1/230,000 [2,3], it is estimated that there are a considerable number of patients who have not received a definitive diagnosis or who have spent all their lives without symptoms of citrin deficiency.