One method for genome editing, called CRISPR, is a process first discovered in immune defense in prokaryotes. This process involves an endonuclease enzyme, , that is capable of identifying sequences of DNA. The enzyme uses a(n) that complementary base pairs with a(n) . In prokaryotes, this method protects from invading viruses. After the DNA sequence is identified, produces a break in both strands of the viral DNA. A strand of DNA adjacent to the target sequence, called , protects the host DNA from damage. This process can be replicated to target specific DNA for . The break in the DNA can be used to a gene to study its role in a cell, or to at specific genomic locations.