Explains the principles of southern blotting, a laboratory technique used to achieve a permanent visible picture of bands of DNA fragments. Follows the step-by-step process from separation through to fixing and to the final visualising of the DNA fragments as bands on a photographic film. Charts the progress of the experiment to compare DNA samples from five individuals, one normal and four with a neurological disease. Professor Alan Lehmann summarises the final autoradiogram. Also describes the northern blotting process, a similar technique for RNA fragments.