INTRODUCTION
Precise genome modification with engineered, site-directed nucleases such as meganucleases, zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and Cas9/sgRNAs has emerged as a powerful genome-enabling tool for fundamental and applied biological research. These technologies rely on DNA double strand breaks (DSBs) generated in a targeted fashion in the genomic region of interest by the potent nucleases (1–3). DSBs stimulate cellular DNA repair mechanisms and are repaired through error-prone nonhomologous end joining (NHEJ) and/or homologous recombination (HR) in vivo, leading to site-specific genetic alterations (4–7). NHEJ has been exploited to generate mutagenic insertions/deletions in transcription
regulatory or protein-coding region of genes of interest that often result in gene inactivation. HR-associated molecules drawn to a specific DSB created by a site-specific engineered nuclease have been harnessed to allow incorporation of desired small or large genetic elements that are flanked by DNA sequences homologous to regions present on either side of the DSB. In most cases, NHEJ events are more predominant than HR in eukaryotic cells and thus are associated with the large majority of targeted genomic events (8–10). To generate a site-specific DSB in
the genomic region, meganucleases, ZFNs and TALENs were developed successively (11–15). Both customizable hybrid nucleases are composed of programmable, sequencespecific
DNA-binding modules fused with the non-specific cleavage domain of FokI nuclease. Such molecules, acting in pairs that recognize adjacent DNA sequences, have enabled precise genomic modifications in a broad range of species, including yeast, plants and animals [reviewed by (2,3)]. However, they often are effectively unavailable for routine use in many laboratories because of the time consuming and labor-intensive process needed for engineering the nucleases genes; furthermore, their sensitivity to the DNA methylation state of the target sites limits the application of those nucleases in genome-wide analysis due to the widespread existence of cytosine methylation in complex genomes (16,17).