The polymerase chain reaction (PCR; 1 ) employs a pair of oligonucleotide primers, one sense and one antisense, with respect to the template DNA. During repeated cycles of heat denaturation, annealing of the oligonucleotide primers to the template and extension of the primers by the enzyme taq polymerase, the intervening fragment is amplified. The use of this thermostable polymerase has led to the automation and subsequent widespread use of this technique. This technique has a number of different applications (2 ) and, in particular, provides a powerful alternative approach to conventional library screening.