DNA polymerase beta, also known as POLB, is an enzyme present in eukaryotes. In humans, it is encoded by the POLB gene. In eukaryotic cells, DNA polymerase beta (POLB) performs base excision repair (BER) required for DNA maintenance, replication, recombination, and drug resistance. The mitochondrial DNA of mammalian cells is constantly under attack from oxygen radicals released during ATP production. Mammalian cell mitochondria contain an efficient base excision repair system employing POLB that removes some frequent oxidative DNA damages. POLB thus has a key role in maintaining the stability of the mitochondrial genome. An analysis of the fidelity of DNA replication by polymerase beta in the neurons from young and very aged mice indicated that aging has no significant effect on the fidelity of DNA synthesis by polymerase beta. This finding was considered to provide evidence against the error catastrophe theory of aging.