Figure 1.

Mechanism of genome editing tools. These genome editing tools employ sequence-specific nucleases to recognize a particular DNA target sequences to generate double stranded breaks (DSBs). zinc finger nucleases (ZFNs) are targetable DNA cleaving proteins applied to cleave DNA sequences at any site. TALENs trigger DSBs at target site that induces DNA damage response pathways, leading to genome modification. CRISPR-Cas9 is an RNA-guided endonuclease directed by guide RNA (gRNA), and it binds at the target site adjacent to the protospacer adjacent motif (PAM) and creates a DSB. To repair the DSBs, two repair mechanisms are used by the cell: non-homologous end joining (NHEJ), which creates indels leading to a loss-of-function mutation, and homology-directed repair (HDR), which involves the introduction of a template DNA to repair DSBs that results in the correction of pre-existing mutations.