CRISPR (which is an acronym for clustered regularly interspaced short palindromic repeats) is a family of DNA sequences found in the genomes of prokaryotic organisms such as bacteria and archaea. These sequences are derived from DNA fragments of bacteriophages that had previously infected the prokaryote. They are used to detect and destroy DNA from similar bacteriophages during subsequent infections. Hence these sequences play a key role in the antiviral (i.e. anti-phage) defense system of prokaryotes and provide a form of acquired immunity. CRISPR are found in approximately 50% of sequenced bacterial genomes and nearly 90% of sequenced archaea.

Cas9 (or "CRISPR-associated protein 9") is an enzyme that uses CRISPR sequences as a guide to recognize and cleave specific strands of DNA that are complementary to the CRISPR sequence. Cas9 enzymes together with CRISPR sequences form the basis of a technology known as CRISPR-Cas9 that can be used to edit genes within organisms. This editing process has a wide variety of applications including basic biological research, development of biotechnological products, and treatment of diseases. The development of the CRISPR-Cas9 genome editing technique was recognized by the Nobel Prize in Chemistry in 2020 which was awarded to Emmanuelle Charpentier and Jennifer Doudna.

CRISPR technology has been applied in the food and farming industries to engineer probiotic cultures and to immunize industrial cultures (for yogurt, for instance) against infections. It is also being used in crops to enhance yield, drought tolerance and nutritional value.

By the end of 2014 some 1000 research papers had been published that mentioned CRISPR. The technology had been used to functionally inactivate genes in human cell lines and cells, to study Candida albicans, to modify yeasts used to make biofuels and to genetically modify crop strains. Hsu and his colleagues state that the ability to manipulate the genetic sequences allows for reverse engineering that can positively affect biofuel production CRISPR can also be used to change mosquitos so they cannot transmit diseases such as malaria. CRISPR-based approaches utilizing Cas12a have recently been utilized in the successful modification of a broad number of plant species.

In July 2019, CRISPR was used to experimentally treat a patient with a genetic disorder. The patient was a 34-year-old woman with sickle cell disease. In February 2020, progress was made on HIV treatments with 60-80% of the DNA removed in mice and some being completely free from the virus after edits involving both LASER ART, a new anti-retroviral therapy, and CRISPR. In March 2020, CRISPR-modified virus was injected into a patient's eye in an attempt to treat Leber congenital amaurosis.

In the future, CRISPR gene editing could potentially be used to create new species or revive extinct species from closely related ones.