Illustration of a HindIII restriction enzyme from the bacterium Haemophillus influenzae bound to its target DNA (deoxyribonucleic acid, yellow). Restriction enzymes, also known as restriction endonucleases recognise specific nucleotide sequences in a DNA strand and make a cut at that site. They are found in bacteria and archaea and are though to have evolved as a defence against viral infection. They are widely used as a tool in genetic engineering.