With the use of ribonuclease A (RNase A) as promoter, only one strand of peptide nucleic acid (PNA) efficiently invades double-stranded DNA (dsDNA), according to the Watson Crick rule. The RNase A locally rewinds the dsDNA to facilitate unimolecular invasion, which otherwise hardly occurs. Furthermore, the single-stranded portion, formed at the invasion site in the dsDNA, is trapped by the protein.