Some genetic mutations occur from exposure to external agents called mutagens, such as radioactivity, ultraviolet radiation from the sun, and certain natural and human-made chemicals.
A mutagen refers to all physical or chemical agents that change the genetic material, of an organism and thus increases the frequency of mutations. Mutagens may be of physical (like ionizing radiation, UV), chemical (reactive oxygen species, deaminating, alkaloids, bromines) or biological origin (viruses, bacteria, transposons).