Disclosed is a method for the detection of a primer extension product produced in the presence of a polymerase lacking 3&rsquo；->；5&rsquo； exonuclease activity, the method comprising the steps of: a) providing at least two single-labelled oligonucleotide sequences that hybridise to one another in free solution to form a fluorescent quenched pair, that upon introduction of a complementary sequence to one or both sequences generates a measurable signal when the complementary sequence is hybridised to one of the at least two single-labelled oligonucleotide sequences, wherein at least one of the oligonucleotide sequences contains at least one phosphorothioate group； b) providing at least one primer and initiating the primer extension reaction from the at one primer thereby generating a complementary sequence to at least one of the single-labelled oligonucleotide sequences； and c) measuring the detectable signal that is generated when the complementary sequence is hybridised to one of the at least two single-labelled oligonucleotide sequences.
