EST (expressed sequence tag): A unique stretch of DNA within a coding region of a gene that is useful for identifying full-length genes and serves as a landmark for mapping. An EST is a sequence tagged site (STS) derived from cDNA.

An STS is a short segment of DNA which occurs but once in the genome and whose location and base sequence are known. STSs are detectable by the polymerase chain reaction (PCR), are helpful in localizing and orienting mapping and sequence data, and serve as landmarks in the physical map of the genome.