NgAgo： an exciting new tool for genome editing

Site-directed DNA endonucleases are powerful tools for genome editing. When introduced into cells, these systems can bind to a target DNA sequence in the genome and create a DNA double-strand break （DSB）, the repair of which leads to varied DNA sequence modifications. The initial efforts on developing these tools were focused on engineering homing endonucleases [1] and zinc finger nucleases （ZFN） [2, 3], then transcription activator-like effector nucleases （TALENs） [4-6]. All these systems rely on designing protein module to recognize specific DNA sequences. Emerged from the acquired immune system of bacteria and archaea, clustered regularly interspaced short palindromic repeats （CRISPR） and the CRISPR-associated proteins （Cas） system is an RNA-guided DNA endonu- clease system in which Cas9 endonuclease is directed to specific genomic locus by a guide RNA that is comple- mentary to target DNA sequence [7, 8]. Due to the sim- plicity of RNA-guided nature and robustness of the system, CRISPR-Cas9 rapidly became the method of choice for genome editing since 2012.