Sequence tagged site (STS): A short (200 to 500 base pair) DNA sequence that occurs but once in the genome and whose location and base sequence are known.

STSs are detectable by polymerase chain reaction (PCR), are useful for localizing and orienting the mapping and sequence data, and serve as landmarks on the physical map of a genome.

Expressed sequence tags (ESTs) are STSs derived from cDNAs (complementary DNAs).