A polyamide backbone, e.g. (-NH-CH₂-CH₂-NR-CH₂-CO-)_n, which bears purine and pyrimidine bases (R in the formulation) and can mimic oligonucleotides by hybridising with ssDNA or RNA. Some PNAs can even form triplexes with DNA strands, and can invade a dsDNA strand to dissociate the base-pairing of the DNA and form a duplex with one strand. PNAs are uncharged, and therefore form more stable complexes with a single stranded nucleic acid, especially at low ionic strength, than the corresponding double-stranded nucleic acid.