This technique, named after its inventor E.M. Southern, is a way of combining restriction enzyme and hybridization information. In Southern blotting, a DNA is digested with a restriction enzyme and the fragments are separated by gel electrophoresis. The DNA fragments are denatured by soaking the gel in base and then are transferred to a piece of nitrocellulose filter paper by capillary action. The result is like a “contact print” of the gel; each fragment is at a position on the paper that corresponds exactly to its position in the gel. Then the filter paper is mixed with a radioactively labeled single‐stranded “probe” DNA or RNA. The probe DNA hybridizes to the complementary single‐stranded DNA fragments on the filter paper. The blot is exposed to X‐ray film and the resulting autoradiograph shows the positions of the complementary fragments of DNA. This information physically maps the regions of a large DNA that are complementary to a probe. See Figure 1.