In the past, 20 metabolic centers in five European countries shared their experience with patients presenting with non-classical UCD, including data from 208 patients with non-classical UCD and deficiencies of ornithine transcarbamylase (OTC; 58%), argininosuccinate synthetase (20%), argininosuccinate lyase (15%), arginase 1 (4%), carbamoyl phosphate synthetase 1 (1%), N-acetylglutamate synthase (1%), and the hyperornithinemia-hyperammonemia-homocitrullinuria syndrome (1%) [21]. Here, ARG1 is linked to urea cycle disorder.