PCR depends on generation of a deoxyoligonucleotide complementary to the known 3' terminus of the portion of DNA to be amplified. This is then annealed with the appropriate strand of heat-denatured DNA and used as a primer for synthesis by a DNA polymerase of a full-length complementary strand (Fig. 1). The two strands are then separated by heat denaturation and the process repeated. However, included within the incubation mixture is a second primer capable of hybridizing to the 3' end of the complementary strand of DNA. Thus, during cycles of denaturation, annealing and DNA extension, both the forward and reverse strands of the initial template DNA can act as templates for synthesis of new complementary strands. Since at the end of a cycle, each of the original complementary strands has generated its own complement, each