CRISPR/Cas systems are employed by bacteria and archaea as a defense against invading viruses and plasmids. Recently, the type II CRISPR/Cas system from the bacterium Streptococcus pyogenes has been engineered to function in eukaryotic systems using two molecular components: a single Cas9 protein and a non-coding guide RNA (gRNA).The Cas9 endonuclease can be rendered inactive (dCas9) with mutations to the two protein domains, RuvC and HnH (D10A and H840A respectively), which are responsible for nuclease activity. The nuclease deficient protein can then be programmed with a gRNA and directed to bind at a desired sequence of DNA. Variants of programmable endonucleases are often unable to cleave a large number of the targets that are efficiently cleaved by canonical SpCas9 in human cells. Cotargeting dead Cas9 may alter local chromatin structures and render otherwise inaccessible target sites now accessible and cleavable by alternative nucleases such as type II-B FnCas9, type II-C CjCas9 and NcCas9 and type V FnCpf1.