Restriction enzymes are proteins that are produced by bacteria as a defense mechanism against viruses that infect the bacteria (bacterial phages). Most bacteria have restriction modification systems that consist of methylases and restriction enzymes. In such systems a bacteria's own DNA is modified by methylation (the addition of a methyl group, CH₃) at a specific location determined by a specific pattern of nucleotide residue and protected from degradation by specialized enzymes termed endonucleases.

The names of restriction enzymes are created from the first letter of the bacterial genus followed by the first two letters of the species plus a Roman numeral if more than one restriction enzyme has been identified in a particular species. Thus, the fifth restriction enzyme from E. coli is called EcoRV (pronounced e-ko-r-five). Besides cloning, restriction enzymes are used in genetic mapping techniques, linking the genome directly to a conventional genetic marker...