Another approach is the use of restriction endonucleases. These are enzymes that cleave DNA at sites specifically related to the nucleotide sequences. Using enzymes with different specificities, a DNA fragment containing a particular gene can be cut out from the DNA molecule. In the Southern blot, these fragments of DNA are electophoresed on agarose gels with the smaller fragments migrating further than the larger ones. Alkaline denaturation of these fragments uncoils the fragment so that a single stranded DNA will hybridize with the complementary DNA after transfer to a special nitrocellulose filter “fixes” these DNA fragments after the electrophoresis. Finally, a radiolabeled probe containing DNA known to be complementary to the DNA of interest hybridizes to it, and the fragment can be identified by autoradiography of the filter. The Northern blotting technique is very similar to the DNA experiment but uses RNA instead of DNA in the system.