Xanthine dehydrogenase (XDH, EC 1.1.1.204) is a molybdenum iron-sulphur flavin hydroxylase which oxidizes a variety of purines, pterins and other heterogenic nitrogen compounds, serving as a rate-limiting enzyme in nucleic acid degradation. In this work, we have isolated and sequenced cDNA clones of human liver XDH. The obtained cDNA covers 4577 bases of human liver XDH mRNA with a 63 bp 5'-end untranslated region and a 515 bp 3'-end untranslated region. A termination codon TGA and a polyadenylation signal AATAAA were identified. An open reading frame encodes 1333 amino acid residues. The assignment of the N-terminal was confirmed by directly sequencing that region of purified human milk XDH. Northern blot analysis shows that the human XDH gene is widely expressed in human tissues.