Chen, Fuqiang, et. a. (2017) Nature Communications. 8:14958. https://www.nature.com/articles/ncomms14958
Many different Cas variants have been discovered, each with slightly different activities. Francisella novicida Cas9 (FnCas9) is one such Cas variant. It is frequently inhibited at genomic loci, presumably due to chromatin formation. By using dead SpCas9 to target proximally, FnCas9 could cleave at the target site, presumably due to chromatin rearrangements.