1.1 Discovery of CRISPR Cas9 gene editing
A mechanism to withstand exogenous viral and plasmid DNA invasion, named CRISPPR Cas9, has emerged from bacteria and archaea below the long-term evolution of nature 9,10. It consists of type 1 (containing I, Ⅲ and IV) and type two (containing II, V and VI), with multisubunit complexes and individual proteins as effectors, respectively 11,12. The type II CRISPR system is the most familiar, whilst crRNA-tracRNA (trans-activated crRNA) is its special structure that can chimerize into single-guided RNA (sgRNA)13-15. DNA endonuclease Cas9 in the type II CRISPR machine acknowledges bound dsDNA substrate and then cleaves the corresponding DNA strand via HNH or RuvC nucleases 16.