Isolated polynucleotide molecules and peptides encoded by these molecules are used in the analysis of human carbamyl phosphate synthetase I phenotypes, as well as in diagnostic and therapeutic applications, relating to a human carbamyl phosphate synthetase I polymorphism. By analyzing genomic DNA or amplified genomic DNA, or amplified cDNA derived from mRNA, it is possible to type a human carbamyl phosphate synthetase I with regard to the human carbamyl phosphate synthetase I polymorphism, for example, in the context of diagnosing and treating hepatic veno-occlusive disease (HVOD) associated with bone marrow transplants.