A technique for detecting specific DNA fragments and analysing their size. For this purpose, genomic DNA is usually cleaved into fragments with restriction enzymes, which are separated according to size by gel electrophoresis and then transferred to a membrane (Southern blot). This membrane is then incubated with a specific, radioactively labeled DNA probe. If the complementary DNA sequence is present on the membrane, the probe attaches to this sequence (hybridization) and can subsequently be detected by autoradiography.