Any agent that causes an increased frequency of mutation. Mutagens are typically short-wave electromagnetic radiations (e.g. ultraviolet irradiation, x-rays, and cosmic rays), ionizing radiations (e.g. α- and β-particles), and chemicals (e.g. nitrous acid and proflavin) that react with nucleotides. A fourth category of mutagens, *base analogues
, do not alter existing bases but are incorporated in place of normal bases during DNA replication. At subsequent replications base analogues may 'mispair', resulting in substitution of the 'wrong' base into DNA, and thus an altered codon. The action of the various types of chemicals and radiation is more variable. Sometimes, as with nitrous acid, the base pairing specificity of the nucleotides is altered, subsequently resulting in incorporation of the 'wrong' base, as with base analogues. Sometimes, as with ultraviolet irradiation, adjacent nucleotides are caused to complex with each other instead of bonding to bases in the complementary helix. This may weaken the double helix and result in chromosome breaks.