Method for detecting a nucleic acid base sequence

A method for the detection of a diagnostic base sequence in nucleic acid in a sample, which method comprises contacting the sample under hybridizing conditions and in the presence of appropriate nucleoside triphosphates and an agent for polymerization thereof, with a diagnostic primer for the diagnostic base sequence, the diagnostic primer having a non-complementary tail sequence comprising a tag region and a detector region, such that an extension product of the diagnostic primer is synthesised when the corresponding diagnostic base sequence is present in the sample, no extension product being synthesised when the corresponding diagnostic base sequence is not present in the sample and any extension product of the diagnostic primer acts as a template for extension of a further primer which hybridizes to a locus at a distance from the diagnostic base sequence, and contacting the sample with a tag primer which selectively hybridizes to the complement of the tag region in an extension product of the further primer and is extended, and detecting the presence or absence of the diagnostic base sequence by reference to the detector region in the further primer extension product.