Found in bacteria and archaebacteria, the regular cluster-interval short palindromic repeat (CRISPR) can effectively resist the invasion and insertion of phage and various gene elements at the gene level. CRISPR-Associate (Cas) protein is a kind of RNA-mediated double-stranded DNA endonuclease which owns the ability to cleave exogenous DNA. When the conserved spacer proto-adjacent motif (PAM) is present in the invading DNA sequence, the target DNA sequence will be cleaved in the spacer sequence upstream (4-5nt) of the PAM. Because the CRISPR-Cas9 system requires only one single Cas9 endonuclease to complete the reaction, and the system is flexible, simple, specific, and low cytotoxicity. It has become a popular genome editing technology and is widely used in genome engineering.
