Cas is short for CRISPR-related proteins. The clustered regularly spaced short palindromic repeats (CRISPR) and associated proteins (Cas) constitute the CRISPR-CAS system, which is a phage-resistant immune system found in many bacteria and most archaea. The CRISPR-Cas system recognizes foreign nucleic acids and cuts them off, thereby silencing the expression of foreign genes. Due to its precise targeting function, the system has been developed as an efficient gene editing tool. At present, the most commonly used Cas nucleases include SpCas9, SaCas9, Cas12, Cas13, etc.