Expressed Sequence Tags (ESTs) are short fragments of DNA that represent expressed genes. Synonyms for ESTs include Expressed Gene Tags, Transcript Tags, and cDNA clones. ESTs were first introduced in the early 1990s as a way to rapidly identify genes of interest, and have since become a valuable resource for studying gene expression in different tissues and under different conditions. Other synonyms for ESTs include Expressed Sequence Clones, mRNA tags, and Expressed Fragment Length Polymorphisms (EFLPs). They have played an important role in genome annotation, gene discovery, and comparative genomics, and continue to be widely used in molecular biology research.