A method is disclosed for detecting single-stranded target nucleic acid (2) which comprises the steps of forming a hybrid between said target nucleic acid and a nucleic acid probe (4), said nucleic acid probe labelled with an enzyme reagent (6) which hydrolyses single-stranded nucleic acid but is substantially without effect on double-stranded nucleic acid, said hybrid formed under conditions of pH which are outside the activity range of said enzyme reagent, adjusting said pH to a value within the activity range of said enzyme reagent allowing said enzyme reagent substantially to hydrolyse any single-stranded nucleic acid present, and detecting said hybrid.
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