Inclusion Criteria:  Diagnosis of primary urea cycle disorder based on clinical suspicion confirmed by enzyme activity, DNA testing or metabolite analysis. History of liver transplantation and/or liver biopsy OR Planned liver transplantation and/or liver biopsy  Exclusion Criteria:  Unavailability of histopathology report from the liver biopsy or explant, or unavailability of liver tissue or slides from the biopsy or explant OR Anticipated inability to obtain pathology report, liver disease, tissue blocks, or pathology slides after liver biopsy or transplantation Known history of a secondary cause for liver disease such as chronic viral hepatitis, autoimmune liver disease, short gut, small bowel syndrome, alcohol liver disease, or TPN-related cholestatic disease