However, in situations where this becomes less practical (e.g., multiplexing GUARD RNAs), co-delivery with active Cas9 will be more appropriate. Many of our experiments involved co-transfection of gRNA and GUARD RNAs in Cas9-expressing cells lines, with no apparent consequences of guide-swapping 26. The design principles of CRISPR GUARD could be easily adapted for use with Cas9 orthologues, TALENs and zinc-finger nucleases, while Cas9 variants that utilise distinct PAMs will increase the number of suitable GUARD RNAs for each off-target. By demonstrating that we can also block editing at perfectly complementary on-target sites (Fig. 2d ), we open the possibility for controlling the kinetics of gene editing by introducing GUARD RNAs as an off-switch. In summary, CRISPR GUARD has exciting potential for improving the specificity and safety of genome editing.