The liver of a young girl which had been successfully transplanted was investigated at the ornithine transcarbamylase (OTC, EC 2.1.3.3) gene expression level. Northern blot hybridization using a human OTC cDNA probe showed a greater than 80% decrease in specific OTC mRNA although having the same molecular size as a normal control. OTC polypeptide was simultaneously synthesized with a normal molecular size but at a low level (20%) as shown by immunoblotting. The OTC enzyme from the deficient liver exhibited very little catalytic activity (7.2% as compared to the normal subject). These results may support several explanations of this disease such as mutation of the OTC gene promoter leading to a low transcriptional activity or mutation of the encoding sequence which results in a modified translation product but with a normal size. mRNA instability may also occur.