mutagen

A mutagen is anything in the environment that raises the rate at which mutations occur, above the natural background level. If spontaneous mutations are the occasional copying slip of a careful scribe, a mutagen is like working in poor light or with a smudged pen — mistakes pile up faster.

Mutagens come in several kinds. Physical mutagens include ultraviolet light and ionising radiation, which damage DNA directly. Chemical mutagens include certain compounds in tobacco smoke and some industrial chemicals that react with or mimic the DNA bases. Some viruses can also disrupt the genome. Many, though not all, mutagens are therefore also potential carcinogens.

It is worth being precise: a mutagen increases the probability of mutation, it does not choose which gene is hit. The damage is essentially random across the genome. This is also why mutagens are deliberately used in the laboratory to generate new variants for genetic screens, a long-standing and powerful research tool.