CRISPR was adapted from a natural bacterial immune system that protects against viruses.

CRISPR originally comes from bacteria and archaea, which use it as a defense against invading viruses. These organisms store snippets of viral DNA in their genome as a memory. When the virus attacks again, the CRISPR system uses this memory to guide Cas proteins to cut the viral DNA, stopping the infection. Scientists realized they could harness this natural system to edit genes in other organisms, including humans, by designing guide RNAs for any DNA target.