sperm DNA for blocking of the membrane surface and target DNA, deionized formamide, and detergents such as SDS to reduce non-specific binding of the probe. Detection: After hybridization, excess probe is washed from the membrane (typically using SSC buffer), and the pattern of hybridization is visualized on X-ray film by autoradiography in the case of a radioactive or fluorescent probe, or by development of color on the membrane if a chromogenic detection method is used. Interpretation of results Hybridization of the probe to a specific DNA fragment on the filter membrane indicates that this fragment contains DNA sequence that is complementary to the probe. The transfer step of the DNA from the electrophoresis gel to a membrane permits easy binding of the labeled hybridization probe to the size-fractionated DNA. It also allows for the fixation of the target-probe hybrids, required for analysis by autoradiography or other detection methods. Southern blots performed with restriction enzyme-digested genomic DNA may be used to determine the number of sequences (e.g., gene copies) in a genome. A probe that hybridizes only to a single DNA segment that has not been cut by the restriction enzyme will produce a single band on a Southern blot, whereas multiple bands will likely be observed when the probe hybridizes to several highly similar sequences (e.g., those that may be the result of sequence duplication). To improve specificity and reduce hybridization of the probe to sequences that are less than 100% identical, the hybridization parameters may be changed (for instance, by raising the hybridization temperature or lowering the salt content). Nylon membrane is more durable and has higher binding capacity to DNA fragments than nitrocellulose membrane, so the DNA fragments will be more fixed to the membrane even when the membrane is incubated in high temperatures. In addition, compared to the nitrocellulose membrane which requires a high ionic strength buffer to bind the DNA fragments to the membrane, nylon charged membranes use buffers with very low ionic strength to transfer even small fragments of DNA of about 50 bp to the membrane, usually the DNA to be transferred is separated by polyacrylamide gel. In the blotting step the most efficient method to transfer the DNA from the gel to the membrane is the vacuum transfer since it transfers the DNA more rapidly and quantitatively. Applications Southern blotting transfer may be used for homology-based cloning based on amino acid sequence of