Mutagen
In biology, a mutagen (Latin, literally origin of change) is a physical or chemical agent that changes the genetic information (usually DNA) of an organism and thus increases the frequency of mutations above the natural background level. As many mutations cause cancer, mutagens are typically also carcinogens. Not all mutations are caused by mutagens: So-called "spontaneous mutations" occur due to errors in DNA replication, repair and recombination of DNA sequences.

# Effects of mutations
The changes in nucleic acid sequences by mutations include substitution of nucleotide base-pairs and insertions and deletions of one or more nucleotides in DNA sequences. Although many of these mutations are lethal, or cause serious disease, some have minor effects, as the changes they cause in the sequence of encoded proteins are not significant. Many mutations cause no visible effects at all, either because they occur in introns, or because they do not change the amino-acid sequence, due to redundancy of codons.

# Genetic drift
The change in a populations genetic material due to the accumulation of random chance is called drift and serves as a molecular clock. In general, the more nucleotide differences between two organisms, the more time has elapsed since their last common ancestor. Though it is difficult to determine in many organisms, estimates for mutation rates have been made for both E. coli and eukaryotes.  It was estimated that in these organisms about one nucleotide in every 1010 is changed and continues through reproduction to future generations of cells.

# Discovery of mutagenesis
In the 1920s, Hermann Muller discovered that x-rays caused mutations in fruit flies.  He went on to use x-rays to create Drosophila mutants that he used in his studies of genetics.  He also discovered that x-rays did not only mutate genes in fruit flies, but also had effects on the genetic makeup of humans.

# Nature of mutagens
Mutagens are usually chemical compounds or ionizing radiation.  Mutagens can be divided into different categories according to their effect on DNA replication:
- Some mutagens act as base analogs and get inserted into the DNA strand during replication in place of the substrates.
- Some react with DNA and cause structural changes that lead to miscopying of the template strand when the DNA is replicated.
- Some work indirectly by causing the cells to synthesize chemicals that have the direct mutagenic effect.
The Ames test is one method to determine how mutagenic an agent is.

# Examples
- Nitrous acid, (HNO2) Deaminating agent
- UV Radiation, thymine dimer formation
- Sodium azide,  (NaN3)
- Gamma and Alpha Radiation, ionising radiations
- Transposons, autonomous DNA fragment relocation/multiplication
- Base analogues, substitutes
- Bromine and some of its compounds,
- Ethidium bromide (EtBr), and other intercalating agents.
- Bromouracil,Alkylating Agent
- Vinca Alkaloids, Natural Plant Alkaloid

# Mutagens in fiction
In science fiction, mutagens are often represented as substances that are capable of completely changing the form of the recipient.
- The Inhumans of Marvel Comics utilize a mutagen called the "Terrigen Mist".
- The Teenage Mutant Ninja Turtles were supposedly created by means of mutagens, as well as their master Splinter and (in the 1987 cartoon) their enemies Bebop and Rocksteady.
- In the Street Fighter movie, a bag of mutagens is used by General Bison to transform Carlos Blanka into a monster.
- In Lego's Bionicle franchise, the water surrounding the pit is mutagenic, causing mutation in most of the major characters who enter the water.