Repairing The Damage
DNA is constantly being damaged, and it is constantly being repaired as well. It is only when the damage is not repaired that a mutation can lead to cancer or cell death. The DNA repair enzymes can recognize damaged nucleotides and remove and replace them. The human liver contains a large number of enzymes whose role is to detoxify toxic compounds, mutagenic or otherwise, by chemically reacting them. However, in some cases these enzymes (called cytochrome P450s) actually create mutagens during the course of these reactions. Such "bioactivation" may be a significant source of mutagens.
Philp, Richard B. Ecosystems and Human Health: Toxicology and Environmental Hazards,2nd ed. Boca Raton, FL: Lewis Publishers, 2001.
Brusick, David. Principles of Genetic Toxicology, 2nd ed. New York: Plenum Press, 1987.