Slaymaker, I.M. et al. Science 351, 84–88 (2016).

Kleinstiver, B.P. et al. Nature 529, 490–495 (2016).

Lack of specificity and off-target cleavage are ongoing problems for the CRISPR-Cas9 gene editing system. Two groups have now made modifications to Streptococcus pyogenes Cas9 to increase its specificity without affecting its activity. Slaymaker et al. designed enhanced specificity Cas9 (eSpCas9), which has weakened interaction between Cas9 and the DNA strand that is not bound by the single guide RNA (sgRNA). eSpCas9 requires stronger base pairing between the target strand and the sgRNA-Cas9 complex to achieve cleavage. Kleinstiver et al. developed a high-fidelity Cas9 (SpCas9-HF) with weakened interaction between Cas9 and the phosphate backbone of the target DNA strand to decrease the energy of the complex and shift its activity to target sites only. Both approaches led to highly specific genome editing tools.