FIGURE 1.

CRISPR/Cas9 system and DSB repair. (A) Mechanism of the CRISPR-Cas9 system. A short guide sgRNA associates with the Cas9 endonuclease to form the Cas9-sgRNA complex. Cas9 is a targeted DNA by PAM under the sgRNA. (B) cNHEJ : cNHEJ was the predominant pathway for repairing DSBs and was used for re-ligating broken DNA ends. Deletions or insertion mutations lead to gene frame shift mutations or premature generation of stop codes. (C) HDR: HDR was used a homologous DNA template to guide DSB repair. The DNA donor templates of HDR were used to insert or replace specific sequences into the genome. (D) MMEJ: MMEJ-mediated repair was capable of generating precise deletions between two short micro-homologous sequences (5–25 base pairs) at target loci.