The starting pool of DNA was prepared by PCR using the synthetic oligomer 5???-GTGCCAAGCTTACCG-N50-GTCGCCATCTCTTCC-3??? (SEQ ID NO 4), where N is an equimolar mixture of G, A, T and C. A 2-ml PCR, containing 500 pmoles of the randomized oligomer, 1,000 pmoles primer 1 (5???-GTGCCAAGCTTACCG-3???, SEQ ID NO 10), 500 pmoles primer 2 (5???-CTGCAGAATTCTAATACGACTCACTATAGGAAGAGATGGCGAC-3???, SEQ ID NO 11