Fig. 1.

The basic working principle of major genome-editing technologies. Meganucleases are engineered restriction enzymes that recognize long stretches of DNA sequences. Each zinc finger nuclease recognizes triple DNA code whereas each TALE recognizes an individual base. Unlike protein–DNA recognition in ZFNs and TALENs, simple RNA–DNA base pairing and the PAM sequence determine CRISPR targeting specificity. All these tools result in DNA double-strand breaks, which are repaired either by error-prone non-homology end joining or homology-directed repair. While NHEJ results in random indels and gene disruption at the target site, HDR can be harnessed to insert a specific DNA template (single stranded or double stranded) at the target site for precise gene editing