Thus, the starting pool of DNA described herein was prepared by PCR using the synthetic oligomer 5 GGG ACG AAT TCT AAT ACG ACT CAC TAT rA GG AAG AGA TGG CGA CAT CTC N40GT GAC GGT AAG CTT GGC AC 3??? (SEQ ID NO 23), where N is an equimolar mixture of G, A, T and C, and where the DNA molecules were selected for the ability to cleave the phosphoester following the target rA. (See FIG. 6A, also.)