Kiani, S. et al. Nat. Methods 12, 1051–1054 (2015).

Dahlman, J.E. et al. Nat. Biotechnol. 33, 1159–1161 (2015).

Researchers use the nuclease activity of Cas9 for genome editing and a nuclease-dead Cas9 mutant for gene activation and repression. Two groups now independently show that shorter guide RNAs (gRNAs) of 14 nucleotides (nt), while still targeting Cas9, fail to activate its nuclease domain. Kiani et al. used gRNAs of both lengths to target wild-type Cas9 fused to a transcriptional activator and to construct kill switches; 14-nt gRNAs activated gene expression of a reporter gene, and inducible 20-nt gRNAs 'killed' the activity of the shorter guides by cleaving the promoter of the reporter. Dahlman et al. observed robust gene activation but no nuclease activity when wild-type Cas9 was targeted by 14-nt gRNA fused to MS2 binding loops, which recruit MS2 protein fused to a transcriptional activator.