Arber hypothesized that bacterial cells might express two types of enzymes: a restriction enzyme that recognizes and cuts up the foreign bacteriophage DNA and a modification enzyme that recognizes ...

Various types of endonucleases – enzymes that can cut DNA – were already known before CRISPR-Cas9. The discovery of restriction enzymes in the early 1970s heralded a new age in molecular biology.

Today, scientists recognize three categories of restriction enzymes: type I, which recognize specific DNA sequences but make their cut at seemingly random sites that can be as far as 1,000 base ...