A technique for detection of mutations in a defined DNA sequence. A labelled PCR-amplified segment of DNA is heat-denatured then quenched at 08C, and the resultant single-stranded polynucleotides are electrophoretically separated on non-denaturing gels. Intrachain base pairing results in a limited number of conformers stabilised by intrachain loops, and mutated DNA shows on electrophoresis an altered assortment of such conformers.