A technique developed by E. M. Southern for transferring DNA fragments isolated electrophoretically in an agarose gel to a nitrocellulose filter paper sheet by capillary action. The DNA fragment of interest is then probed with a radioactive nucleic acid probe that is complementary to the fragment of interest. The position on the filter is determined by autoradiography. The related techniques for RNA and proteins have been dubbed ‘Northern’ and ‘Western’ blots, respectively.