A mutagen is an agent that causes changes in DNA sequences, which can be very detrimental to an organism. In severe cases, exposure to mutagens can result in mutations that lead to cancer or cell death. Mutagenes can be of either physical, chemical or biological origin. Ultraviolet radiation is an example of a physical mutagen, where nucleotide bases exposed to high levels of UV radiation can form pyrmidine dimers, which in turn cause errors during replication and transcription. There are also numerous types of chemical mutagens, such as reactive oxygen species, alkylating and de-aminating agents, as well as certain industrial solvents. Some biological mutagenes, such as viruses (f.e. retroviruses, adenoviruses, HPV), can also cause DNA damage.

Some mutagens (such as acetamines or nitrosamines) require metabolic activation first, to have their negative effect on DNA. These are called promutagenes.

It is important to note that not all mutations are caused by mutagens; many arise spontaneously through the regular cellular processes of DNA replication, repair and recombination.