CRISPR technology affords a simple and robust way to edit the genomes of cells, providing powerful tools for basic research and medicine. While using Cas9 to target a genomic site is very efficient, making a specific mutation at that site is much less so, as it depends on the endogenous DNA repair machinery. Various strategies have been developed to increase the efficiency of knock-in mutagenesis, but often the desired cells remain a small percentage of the total population. To improve efficiency, strategies to select edited cells have been developed. In some applications, a selectable foreign gene is linked directly to the gene of interest (GOI). Alternatively, co-editing, where the GOI is edited along with a selectable gene, enriches the desired cells since the cells that successfully edited the selectable gene are likely to have also edited the GOI. To minimize perturbations of the host genome, "scarless" selection strategies have been developed, where the modified cells are mutated solely in the GOI. In this review, we will discuss strategies employed to improve specific genome editing in mammalian cells, focusing on ways to select successfully edited cells.
Keywords: CRISPR; HDR-dependent editing; co-editing; genome editing; scarless selection.