Figure 1: Schematic representation of the configurations assumed by oligonucleotides with an overlap complementary to a DNA target.

The 3´ end of the upstream oligonucleotide (A) or the 5´ end of the downstream oligonucleotide (C) can each be displaced by the other, with both conformers interchanging through an intermediate form (B). The FEN1 endonucleases used in this study cleave only the flap created by displacement of the downstream segment (C).