During DNA replication, DNA polymerases elongate new strands of DNA against complementary template strands. Most DNA polymerases comprise two different enzymatic domains: a polymerase and a proofreading exonuclease. The polymerase elongates the new strand in the 5' → 3' direction. The exonuclease removes erroneous nucleotides from the same strand in the 3’ → 5’ direction. This exonuclease activity is essential for a DNA polymerase's ability to proofread. Deletions inactivating or removing these nucleases increases rates of mutation and mortality in affected microbes and cancer in mice.