CRISPR is the abbreviation for clustered regularly interspaced short palindromic repeats. It is a family of DNA sequences found in the genomes of prokaryotic organisms such as bacteria and archaea, which plays an important role in the antiviral defense system of prokaryotes. These sequences are derived from the DNA fragments of bacteriophages that have previously infected the prokaryote, and can then be used to detect and degrade DNA from similar bacteriophages during subsequent infections, thus providing the adaptive immunity.

The CRISPR locus has two distinct characteristics, nucleotide repeats and spacers. The size of CRISPR repeats typically ranges from 28 to 37 base pairs, which are distributed throughout the CRISPR region. Spacers in CRISPR are interspersed among these repeats with a size of 32 to 38 base pairs. Sequence of spacers are taken from virus that previously infected the organism, which serve as a bank of memories and can recognize and disable future infection from the same viral strain.