A transposon is a mobile genetic element, capable of excising itself from one position in the genome and reinserting itself in another position with the action of enzymes called transposases. This process is called transposition. Transposons were discovered during work on Maize by Barbara McClintock in the early 1960s. They have since been discovered to comprise a large component of eukaryotic genomes. The vast majority of DNA, previously dubbed 'junk DNA', is the consequence of transposition and we are now aware this has a large role to play in regulating the development of an organism. For instance, repetitive sequences generated by the accumulation of transposons may play a role in the aligning of homologous chromosomes during prophase I of meiosis.

Class I transposons, or retrotransposons, act through reverse transcriptase and an RNA intermediate. Class II tranposons, or DNA transposons, lack an RNA intermediate. Class I transposons are often described as 'copy and paste' transposons because their mode of transposition causes their number to increase in the genome. Class II transposons are often described as 'cut and paste' transposons, because they move around in the genome individually without increasing their numbers.

Both classes of transposon may be mutagenic if they are inserted into the coding region of a gene. However, if the transposon is viewed as an organism and the genome is viewed as its ecosystem, then it is not in a transposon's "interest" to mutate a gene and thereby cause the termination of the genome. There is thus a 'selective pressure' for transposons to be non-mutagenic.