Though it originally appeared that sequencing the human genome would involve the building of ordered sets of cloned DNA fragments, this process involves several steps and is extremely time consuming.  In 1989, Olsen, Hood, and Botstein proposed the universal use of 200-500 nucleotide segments called sequence-tagged sites (STSs) be used as physical landmarks along chromosomes, each set spanning the genome at intervals of about 100 kb.

The polymerase chain reaction (PCR) enables researchers to assay for STSs much more quickly and efficiently, since it requires very little DNA and can produce a million or more fragments in only several hours.