FIGURE 3.

Model of the CRISPR/Cas9 system. The CRISPR/Cas9 gene editing system requires 2 components: the Cas9 nuclease (shown in blue) and the guide RNA (gRNA), comprising of trans-activating CRISPR RNA (tracrRNA) and CRISPR RNA (crRNA) that is complementary to target gene adjacent to the protospacer adjacent motif (PAM, 5′-NGG-3′ where N is non-G). Cas9 complexes with dual gRNA via binding to tracrRNA, whereas crRNA recognition of the target DNA sequence adjacent to the nearby PAM directs Cas9 to induce a double-strand brake (DSB), which is most commonly repaired through the non-homologous end joining (NHEJ) pathway. As error-prone NHEJ repair is not homology-dependent, insertions or deletions (indels) often occur, resulting in disruption and inactivation of the targeted gene.