<p>This domain is responsible for the 3'-5' exonuclease proofreadingactivity of <taxon tax_id="562">Escherichia coli</taxon> DNA polymerase I (polI) and other enzymes, it catalyses the hydrolysis of unpaired or mismatched nucleotides. This domain consists of the amino-terminal half of the Klenow fragment in E. coli polI it is also found in the Werner syndrome helicase (WRN), focus forming activity 1 protein (FFA-1) and ribonuclease D(RNase D) [<cite idref="PUB00003908"/>].</p>