Broadly speaking, any substance, natural or man-made, that results in a mutation that alter the sequence or structure of DNA. Radiation, in the form of x-rays, was the first mutagen to be identified in 1927 by Hermann J. Muller (1890-1967) in fruit flies. Nitrogen mustard (the main component of mustard gas) was then identified as a chemical mutagen in 1942 by Charlotte Auerbach (1899-1994) and John M. Robson (1900-1982), once again in fruit flies. Subsequently, tobacco smoke and even coffee were shown to contain mutagenic agents, the latter in 1986 by means of subcutaneous injections of coffee in rats. Mutagens can be carcinogenic, but not all mutagens are carcinogens. The Ames test is used to screen for chemicals used in foodstuffs and medicines that could be mutagenic agents.
See Carcinogen, DNA (deoxyribenucleic acid), Forward genetics, Gene traps, Mutant, Mutation (biology), Teratogen