The polymerase chain reaction (PCR) is used to amplify a segment of DNA that lies between two regions of known sequence (1–3). It requires two oligonucleotide primers that flank the DNA fragment to be amplified and employs repeated cycles of heat denaturation of the DNA, annealing of the primers to their complementary sequences, and extension of the annealed primers with a thermostable DNA polymerase (4). These primers typically have different sequences, are complementary to sequences that lie on opposite strands of the template DNA, and flank the segment of DNA that is to be amplified. Since the extension products themselves are also complementary to and capable of binding primers, successive cycles of amplification essentially double the amount of target DNA synthesized in the previous cycle.