CRISPR–Cas9 genome-editing method. Schematic representation of how the CRISPR–Cas9 system can be used to introduce a site-specific double-strand break and how the break can be repaired. The Cas9 enzyme, bound to an appropriately designed synthetic guide RNA (sgRNA), will bind to the chromosome around a protospacer-adjacent motif (PAM) site and introduce a double-strand break. By providing a suitable double-strand DNA-repair template with long homology regions on both sides of the double-strand break site, the break will be repaired. In this manner, using a custom template, gene deletions, insertions, and single-base changes can be made. (Reprinted, with permission, from Chen et al. 2017, © American Chemical Society.)