Mutagenic agents, or mutagens, are physical, chemical or biological factors that can cause alteration in DNA molecules.
Examples of well-known or believed to be mutagenic agents are: X, alpha, beta and gamma rays, ultraviolet radiation, nitrous acid, many dyes, some sweeteners, some herbicides, many substances of tobacco, some viruses, like HPV, etc. Small DNA fragments known as transposons can also act as mutagens when incorporated into other DNA molecules.