Chen, F., et. al. (2017) Nature Communications 8:14958. https://www.ncbi.nlm.nih.gov/pubmed/28387220
Streptococcus pyogenes Cas9 (SpCas9) has been the preferred CRISPR/Cas system since its description in 2012, though many other Cas9 variants have been identified. Some of these variants, such as Francisella novicida Cas9 (FnCas9), Campylobacter jejuni Cas9 (CjCas9), and Neisseria cinereal Cas9 (NcCas9) have shown little activity in mammalian systems, presumably due to chromatin structure. By targeting both dead SpCas9 and a second Cas9 variant, Chen et al. could demonstrate activity in cell culture. Chen et al. believe that dead SpCas9 opened the chromatin allowing the second Cas9 access for activity. As each of the different Cas9 variants demonstrate slightly different activities and PAM sequences this technique could greatly expand the CRISPR tool box.