In the early studies, researchers utilized rare cutting endonuclease enzymes, such as the 18-bp cutter I-SceI, to introduce specific DSBs in the mouse genome 10. Therefore, many research groups focused on developing different strategies to achieve targeted DSBs. One of the initial breakthroughs came from the realization that the introduction of a double-strand break (DSB) at a target site results in a several orders of magnitude increase in the frequency of targeted gene integration 9, 10. Researchers sought alternative approaches to overcome these aforementioned limitations. Finally, and most critically, the approach could result in random integration of the exogenous copy into undesired genomic loci at a frequency similar to or higher than that of the target site 8. Secondly, the integration rate depended on cell types and cellular states. Firstly, the rate of spontaneous integration of an exogenous DNA copy was extremely low (1 in 10 3–10 9 cells) 7. However, the feasibility of this approach had several limitations. Such targeted gene integration into the genome provided unprecedented power to characterize the functional roles of various genes in model organisms. Their studies demonstrated that mammalian cells can incorporate an exogenous copy of DNA into their own genome through a process called homologous recombination 5, 6, 7. Initial targeted gene disruption studies in eukaryotic yeast cells 4 followed with breakthrough work by Capecchi and Smithies in mammalian cells 5, 6, 7. To this end, several key developments were revealed in the mid to late 1980s. Although such efforts drove a number of discoveries in molecular biology and genetics, the ability to precisely alter DNA in living eukaryotic cells came a few decades later. For the first time ever, scientists gained the ability to manipulate DNA in test tubes. To this end, the discovery of restriction enzymes that normally protect bacteria against phages in the late 1970s 1, 2, 3 was a turning point that fueled the era of recombinant DNA technology. Therefore, introducing desired changes into genomes, i.e., “genome editing”, has been a long sought-after goal in molecular biology. The ability to change these DNA bases at precisely predetermined locations holds tremendous value not only for molecular biology, but also for medicine and biotechnology. Genomes of eukaryotic organisms are composed of billions of DNA bases. Finally, it will briefly discuss current and future impacts of these tools in science, medicine, and biotechnology. The application areas of CRISPR technology that are extending beyond genome editing, such as targeted gene regulation, epigenetic modulation, chromatin manipulation, and live cell chromatin imaging, will be particularly emphasized. However, for the most part the review will focus on the CRISPR technology. This review will present the brief history and key developments in the field of genome editing and major genome-engineering tools. The genome-editing technologies and CRISPR tools have come to the current exciting stage through years of basic science research and progress from a large number of researchers. During this process, researchers with creative minds and deep background knowledge can seize the opportunity to converge seemingly separate research fields and make a bigger scientific impact. Even so-called serendipitous discoveries come when an inquisitive and open-minded researcher designs a series of careful experiments to follow an interesting observation. They are often built on decades of combined efforts of many great minds. Groundbreaking scientific advancements have several characteristics. This is perhaps true for other scientific disciplines too. However, a closer look at their history reveals that truly serendipitous discoveries are very rare, if not absent in molecular biology. Great inventions and discoveries are often storied as a series of lucky coincidences.
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