This protocol describes a basic method to perform the Southern blot. Blotting allows the detection of specific molecules among a mixture separated by gel electrophoresis. Molecules are transferred from the gel to a porous membrane by capillary action using absorbent paper to soak solution through the gel and the membrane. For DNA, specific sequences are detected in the membrane by molecular hybridization with labeled nucleic acid probes. The original method, on which this protocol is based, used labeled RNAs to detect specific DNA fragments in genomic DNA that had been digested with restriction endonucleases. This protocol can be completed in 1-5 d and is inexpensive to carry out, as it requires only basic laboratory equipment.