Current technology enables the production of highly specific genome modifications with excellent efficiency and specificity. Key to this capability are targetable DNA cleavage reagents and cellular DNA repair pathways. The break made by these reagents can produce localized sequence changes through inaccurate nonhomologous end joining (NHEJ), often leading to gene inactivation. Alternatively, user-provided DNA can be used as a template for repair by homologous recombination (HR), leading to the introduction of desired sequence changes. This review describes three classes of targetable cleavage reagents: zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and CRISPR/Cas RNA-guided nucleases (RGNs). As a group, these reagents have been successfully used to modify genomic sequences in a wide variety of cells and organisms, including humans. This review discusses the properties, advantages, and limitations of each system, as well as the specific considerations required for their use in different biological systems.
Keywords: CRISPR/Cas; DNA repair; HR; NHEJ; TALENs; gene targeting; homologous recombination; nonhomologous end joining; zinc-finger nucleases.