An agent that causes an increase in the number of mutants (see mutation) in a population. Mutagens operate either by causing changes in the base sequence of the DNA of the genes, so interfering with the coding system, or by causing chromosome damage. Various chemicals (e.g. aflatoxin, benzopyrene in cigarette smoke, colchicine), forms of radiation (e.g. X-rays, gamma rays), and retroviruses are among the many agents identified as mutagens.