The DnaQ-like exonuclease superfamily is a structurally conserved group of 3'-5' exonucleases, which catalyze the excision of nucleoside monophosphates at the DNA or RNA termini in the 3'-5' direction. It is also called the DEDD superfamily, after the four invariant acidic residues present in the catalytic site of its members. The superfamily consists of DNA- and RNA-processing enzymes such as the proofreading domains of DNA polymerases, other DNA exonucleases, RNase D, RNase T, Oligoribonuclease and RNA exonucleases (REX). The DnaQ-like exonuclease domain contains three conserved sequence motifs termed ExoI, ExoII and ExoIII, which are clustered around the active site and contain four conserved acidic residues that serve as ligands for the two metal ions required for catalysis. The conservation patterns of the three motifs may vary among different subfamilies. DnaQ-like exonucleases are classified as DEDDy or DEDDh exonucleases depending on the variation of motif III as YX(3)D or HX(4)D, respectively. The significance of the motif differences is still unclear. Almost all RNase families in this superfamily are present only in eukaryotes and bacteria, but not in archaea, suggesting a later origin, which in some cases are accompanied by horizontal gene transfer.