Blotting is the technique for detecting one RNA within the mixture of RNAs (a Northern blot) or one type of the DNA within of a mixture of DNAs (a Southern blot). A blot can prove whether that one species of RNA or DNA is present, how much is there, and its estimated size. Mainly, blotting involves gel electrophoresis, transfer to the blotting membrane (typically nitrocellulose or activated nylon), and incubating with the radioactive probe. Exposing membrane to X-ray film creates darkening at the spot correlating with the position of the DNA or RNA of interest. The darker is the spot, the more nucleic acid was present there.