Yang, H. et al. Cell 154, 1370–1379 (2013).

The bacterial clustered, regularly interspaced, short palindromic repeats (CRISPR)-Cas9 system has previously been harnessed to efficiently mutate multiple genes in the mouse. Yang et al. now extend these studies to demonstrate the precise insertion of small epitope tags, fluorescent protein reporters and flanking loxP sequences into specific target sites in the mouse genome via homology-directed repair. This is achieved by injection of the CRISPR-Cas9 components, including either single-stranded oligonucleotides or double-stranded vectors as donor templates, directly into zygotes. The researchers further conducted an off-target analysis for multiple independent sites and, in contrast with the results of previous studies in human cells, found no modifications at sites with three or more mismatches with the target.