We focused on UCDs where the presentation was more likely to be severe and in the neonatal period: carbamoyl phosphate synthetase 1 (CPS1, OMIM *608307) deficiency (CPS1D, OMIM #23730), ornithine transcarbamylase (OTC, OMIM *300461) deficiency (OTCD, OMIM #311250, and argininosuccinate synthetase (ASS, OMIM *603470) deficiency (ASSD, OMIM #215700). Here, ASS1 is linked to ornithine carbamoyltransferase deficiency.