Amino acid sequence comparisons have revealed that a potential dUTPase gene is encoded by the retrovirus HERV-K, a defective multicopy virus that is transmitted vertically in humans. This gene is distinct from the human cellular dUTPase gene and thus two potential sources of the enzyme exist in human cells. dUTPases characterized from various sources each contain five conserved amino acid sequence motifs that form the active site of the enzyme. The protein sequence of the putative HERV-K dUTPase deduced from previous DNA sequence data from one proviral clone (HERV-K10) shows marked deviations at highly conserved residues in four of five of these motifs. Therefore, the reported DNA sequence may represent a mutated form of the viral dUTPase gene. To address this possibility, we cloned and sequenced 22 copies of the HERV-K dUTPase gene from human DNA. The results of this analysis indicate that variations evident in the HERV-K10 dUTPase amino acid sequence represent mutations of the wild-type viral DNA sequence. A version of the HERV-K dUTPase gene that corresponds to the ancestral, wild-type DNA sequence was constructed and adapted for expression in Escherichia coli. The resulting enzyme was found to exhibit properties similar to those of dUTPases isolated from other systems. A possible role of the HERV-K dUTPase in human disease is discussed.