Expressed sequence tags (ESTs) are DNA sequences that represent the expressed portion of a gene. They include exons (cistrons) but not introns. A cistron is often used as a unit of genetic function because it is a relatively small and discrete unit of DNA that codes for a specific protein. A muton is a unit of DNA that is the smallest unit of genetic material that can undergo a mutation (a single base pair). ESTs are typically short fragments of cDNA that are produced when RNA is reverse transcribed into DNA. They are used as a tool for gene discovery and for identifying and characterising genes.