Abstract: In a matched pair of oligonucleotides (ODNS) each member of the pair is complementary or substantially complementary in the Watson Crick sense to a target sequence of duplex nucleic acid where the two strands of the target sequence are themselves complementary to one another. The ODNs include modified bases of such nature that the modified base forms a stable hydrogen bonded base pair with the natural partner base, but does not form a stable hydrogen bonded base pair with its modified partner. This is accomplished when in a hybridized structure the modified base is capable of forming two or more hydrogen bonds with its natural complementary base, but only one hydrogen bond with its modified partner. Due to the lack of stable hydrogen bonding with each other, the matched pair of oligonucleotides have a melting temperature under physiological or substantially physiological conditions of approximately 40.degree. C. or less.