Fu, Y. et al. Nat. Biotechnol. doi:10.1038/nbt.2808 (26 January 2014).

The recent widespread excitement around the use of the RNA-guided Cas9 endonuclease for targeted genome editing has been tempered by the realization that this tool can generate substantial off-target effects. Fu et al. now show that using slightly shortened guide RNAs (gRNAs), which target the Cas9 nuclease to the desired site in the genome, can help mitigate this problem. By reducing the region of gRNA-target complementarity from 20 nucleotides to 17 or 18 nucleotides, the researchers report that even 1- or 2-nucleotide mismatches can reduce cleavage to very low or even undetectable levels. They also show that truncated gRNAs can be used with paired Cas9 nickases to increase the fidelity of the platform even further.