Structure of the oligonucleotides and relative position of the two mismatches. (A) Sequence and structure of the oligonucleotides modified with a TA-clamp generating a first nucleotide exchange at position 153 (light blue) ore 151 (dark blue) respectively. The possible positions for the second (silent) exchange are given in red. (B) Structure of the oligonucleotides showing the first mismatch at position 153 and the second mismatch at position 147 in various distance from the unmodified 3'-end. (C) Structure of the oligonucleotides with different length, modified with PTO and showing the first mismatch at position 151 and the second mismatch at position 159.