A 2.4 kilobase cDNA clone of human sulfite oxidase was isolated from a human liver cDNA library in lambda gt10. Comparison of three sulfite oxidase sequences to several plant and fungal nitrate reductase sequences reveals a single conserved cysteine with highly conserved flanking sequences. The conserved cysteine is postulated to be a ligand of molybdenum in sulfite oxidase and nitrate reductase.