Chen et. al. (2017) Nature. https://www.ncbi.nlm.nih.gov/pubmed/28931002
Researchers using FRET to study previously engineered high-fidelity Cas9 (SpCas9-HF1) and enhanced Cas9 (eSpCas9), identified that these versions are trapped in an inactive state when bound to off-target sites. Using this observation and rational protein engineering, the researchers made additional modifications to the REC3 domain to prevent activation of the HNH nuclease domain unless the guide RNA and target DNA match is very close. This new Cas9, coined Hyper Cas9 (HypaCas9) maintains the native Cas9 on target efficiency, but decreases the number of off-target events.