This domain is found in a superfamily of enzymes with a predicted or known phosphohydrolase activity [<cite idref="PUB00054925"/>]. These enzymes appear to be involved in the nucleic acid metabolism, signal transduction and possibly other functions in bacteria, archaea and eukaryotes.The fact that all the highly conserved residues in the HD superfamily are histidines or aspartates suggests that coordination of divalent cations is essential for the activity of these proteins [<cite idref="PUB00005837"/>].