Technique used to detect unique DNA sequences within a complex mixture. Briefly, the technique starts by cutting the DNA with specific restriction enzymes and using gel electrophoresis to separate the resulting DNA fragments by size.

After electrophoresis, the gel is briefly treated with an alkaline solution to denature the DNA fragments (i.e. separate them into single strands of DNA). The fragments are then transferred to a nitrocellulose filter paper or a nylon membrane and incubatated with a denatured, labeled DNA fragment (called a "probe") that is complementary to the DNA sequence of interest. The labeled probe should bind to any fragments containing the complementary DNA sequence.

The DNA fragments with the sequence of interest can now be visualized by detecting the probe. If the probe was labeled radioactively (e.g. with 32-P), its presence can be detected by placing a piece of X-ray film on top of the filter or membrane. The radioactive decay particles will result in one or more dark bands, indicating DNA fragments of specific sizes where the probe bound.