[4] 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.

[6][7] Cabelof et al. measured the ability to repair DNA damage by the BER pathway in tissues of young (4-month-old) and old (24-month-old) mice.

[8] In all tissues examined (brain, liver, spleen and testes) the ability to repair DNA damage declined significantly with age, and the reduction in repair capability correlated with decreased levels of DNA polymerase beta at both the protein and messenger RNA levels.

In addition to these cis-regulatory elements present within the 3'UTR a trans-acting protein, HAX1 is thought to contribute to the regulation of gene expression.