A unique short sequence of DNA, typically 200 to 500 nucleotides long, that can be used as a tag, or marker, in physical mapping of cloned DNA segments. An STS is derived by sequencing a short length of a particular clone, then using the sequence data to design primers for the polymerase chain reaction (PCR). Using the primers, PCR will amplify the sequence in every clone containing it. Clones that have a unique sequence in common must overlap. By repeating the process for different STSs, the clones can be aligned into a series of overlapping segments (contigs). See physical map.