A method for characterising a DNA analyte comprised of one or more polynucleotide types characteristic of a site of nucleotide polymorphism each of which includes a target region having the formula -X-Y-Z- wherein X and Z are respectively first and second characteristic flanking oligonucleotide regions and Y is one of the variants constituting the site is provided. The method is characterised by the steps of; (a) reacting a single-stranded oligonucleotide including the target region derived from at least one of the polynucleotide types with a set of unused probes comprised of (i) a single-stranded first aptamer terminating at its 3' end in a sequence complementary to that of -X or –X-Y and (ii) one or more second single-stranded aptamers terminating at their 5' end in a sequence complementary to that of –Z-Y or –Z (as the case may be) and labelled with detectable elements which are in an undetectable state in the presence of a ligase to create a substantially double-stranded used probe comprised of the
