A chromatographic technique for isolating and identifying specific fragments of DNA, such as the fragments formed as a result of DNA cleavage by restriction enzymes. The mixture of fragments is subjected to electrophoresis through an agarose gel, followed by denaturation to form single-stranded fragments. These are transferred, or ‘blotted’, onto a nitrocellulose filter where they are immobilized in their relative positions. Specific gene probes labelled with a radioisotope or fluorescent marker are then added. These hybridize with any complementary fragments on the filter, which are subsequently revealed by autoradiography or a fluorescence detector. The technique was devised by US biologist E. M. Southern (1938– ). A similar technique for detecting RNA fragments is called northern blotting, by analogy. See also western blotting.