CRISPR (Clustered regularly interspaced short palindromic repeats) loci and their Cas (CRISPR associated) genes encode an adaptive immune system that protects bacteria and archaea from viral (phage) and plasmid infection. The RNA-guided DNA endonuclease Cas9 associates with the dual RNA guides (CRISPR RNA [crRNA]) and trans-activating RNA [tracrRNA]) or a synthetic single-guide RNA (sgRNA) and cleaves double-stranded DNA targets complementary to the guide RNA. The double-stranded break occurs 3 bp upstream of the PAM site, allowing targeted sequence modifications via two DNA repair pathways: non-homologous end joining (NHEJ) that introduces frame shift insertion and deletion (indel) mutations leading to loss-of-function alleles and homology-directed repair (HDR) for precise insertion of point mutations or a fragment of desired sequence at the targeted locus. At present, the CRISPR-Cas9 system has been applied to a variety of model organisms, including human, mice, rats, zebrafish, elegans, plants and bacteria.
Genome editing refers to an emerging branch of biotechnology that is earlier genetic engineering technologies. By using these biotechnologies researchers are able to target specific DNA sequences and induce a double stranded break, taking advantage of recombination to create synthetic genetic genome of a host.