Disclosed is a method for the detection of a primer extension product produced in the presence of a polymerase lacking 3’->;5’ 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.