Various sequences such as ESTs, SSLPs and random genomic sequences can be used as a source for STS mapping. Expressed sequence tags (ESTs) are short sequences obtained by analysis of complementary DNA (cDNA) clones. They represent the sequences of the genes being expressed in a cell from which mRNAs are derived. It is commonly used as STS as it comes from a unique gene rather than from a member of a gene family having same or similar gene sequences. Random genomic sequences may be obtained by downloading sequences deposited in the databases or by sequencing random fragments of cloned DNA.

Assembly of fragments (often referred as mapping reagent) can then be performed in two ways- as a clone library or as radiation hybrids.

A clone library can be used as mapping reagent in STS analysis having an average size of several hundred kb represented in Figure 6-5.3(b).3.2 . Chromosome-specific library can be constructed by separating the chromosomes by flow cytometry.

Figure 6-5.3(b).3.2: Clone libraries used in the construction of STS maps and also as a source of DNA to be sequenced. (Source: Brown TA. 2002. Genomes. 2^nd ed. Oxford: Wiley-Liss )