This domain is found in a diverse range of phosphoesterases, including bis(5'-nucleosyl)-tetraphosphatase (apaH), nucleotidases, sphingomyelin phosphodiesterases and 2'-3' cAMP phosphodiesterases, as well as nucleases such as bacterial SbcD or archaeal/yeast Mre11. The most conserved regions in this domain centre around the metal chelating residues.