Hyper-accurate Cas9 The rising popularity of the CRISPR–Cas9 system for genome editing and regulation goes hand in hand with calls for increased specificity and reduced off-target binding. Chen et al. use single-molecule Förster resonance energy transfer to investigate the mechanism by which two recently described highly specific Cas9 variants discriminate between onand off-target sequences. They find that a noncatalytic domain in the Cas9-recognition domain recognizes complementarity between the single-guide RNA and the target DNA duplex and undergoes a conformational change that facilitates the activation of a Cas9 nuclease domain. In the high-fidelity Cas9 variants, this conformational switch is only triggered by a perfect match between target DNA and guide RNA. Based on this insight the researchers then design a hyper-accurate Cas9 (HypaCas9) with high specificity and no loss in on-target activity. Chen, J.S. et al. Nature http://dx.doi.org/10.1038/nature24268 (2017).