Restriction endonucleases can cut DNA at recognition sequences, called restriction sites. A restriction enzyme makes two incisions through the sugar phosphate backbone of DNA in each strand of the double helix. More than 600 of these enzymes are available and originally were discovered in bacteria as part of their defense mechanism against invading viruses. Recognition sites are usually 4-8 nucleotides in length and read the same backwards and forwards. When using the restriction enzyme, the piece of cut DNA has sticky ends. In forensics, this technique can be used to distinguish a person's genotype by digesting the sample with restriction enzymes to generate DNA fragments and then the different sized fragments are separated by gel electrophoresis. Alleles with correct restriction sites will generate two visible bands of DNA on the gel and those with altered restriction sites, will not be cut generating a single band. The number of bands is unique to a person and will reveal his or her unique genotype.