Restriction Endonucleases - Endonucleases are enzymes that produce internal cuts, called cleavage, in DNA molecules. Many endonucleases cleave DNA molecules at random sites. But a class of endonucleases cleaves DNA only within or near those sites, which have specific base sequences; such endonucleases are known as restriction endonucleases, and the sites recognised by, them are called recognition sequences or recognition sites.

The recognition sequences are different and specific for the different restriction endonucleases or restriction enzymes.Restriction enzymes were discovered due to and named after the phenomenon of host restriction of bacterial phages.