Schematic representation of zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein 9 (Cas9) bound to their cognate sites to deliver a targeted genomic double-strand break. (a) ZFNs, (b) TALENs and (c) CRISPR/Cas9. Both ZFNs and TALENs possess a FokI nuclease domain for DNA cleavage, whereas the CRISPR-based guide RNA recruits a Cas nuclease for DNA cleavage. All three systems use components for DNA binding that can be readily engineered for sequence specificity: ZFNs and TALENs possess twin DNA-binding domains, arranged in an inverted dimer, for DNA recognition (zinc finger and transcription activator-like effector domains, respectively); the CRISPR/Cas9 system recognizes DNA through an RNA-DNA interaction between the target site and a CRISPR-based synthetic guide RNA. (d) Targeted genome engineering by non-homologous end joining (NHEJ) or homologous recombination (HR) using the gene editing tools. In the homology-directed repair experiment, cells are co-transfected with both the gene editing tool and a wild-type (WT) or transgene donor DNA.