Tautomerism (from the Greek tauto = same, meros = part) is a special form of isomerism. In tautomerism, there is a chemical equilibrium between two isomeric molecules that have the same molecular formula. However, the molecules can transform into each other through (reversible) proton migration and relocation of a double bond. This results in a chemical equilibrium between the isomers. This type of isomerism is therefore also known as equilibrium isomerism. Under normal conditions, a tautomeric substance always transforms itself into its isomers, the tautomers, and always in the same quantitative ratio. Normally, only one of the two isomers can be isolated from a tautomeric equilibrium, as the removal of a substance from a chemical equilibrium causes its replication, whereby the other isomer is broken down.
The tautomerism of DNA bases is one of the causes of mismatches in DNA replication, such as the keto-enol tautomerism of guanine and thymine or the amino-imino tautomerism of adenine and cytosine. The rare tautomeric form of the bases (the enol form of guanine and thymine or the imino form of adenine) tends to be mismatched during replication and thus contributes to mutations.