The bacterial type II clustered regularly interspaced short palindromic repeat (CRISPR) and CRISPR-associated (Cas) protein system of Streptococcus pyogenes evolved as a component of the prokaryotic immune system (Jinek et al., 2012) and has recently been repurposed for editing of the human genome (Cong et al., 2013). When complexed with an artificial single guide RNA (gRNA) to form an RNA-guided endonuclease, Cas9 can be directed to almost any genomic location, provided that the 20 base pair nucleotide gRNA target sequence satisfies the protospacer-adjacent motif requirement (Cong et al., 2013). Such limited target sequence requirements, together with the simplicity and ease of cloning synthesized gRNAs, has led to novel applications beyond genome editing. By simultaneously introducing nuclease-null mutations into Cas9 (Gilbert et al., 2013, Qi et al., 2013), and coupling the catalytically inactive or dead Cas9 (dCas9) to a variety of effector protein domains, the modulation of transcription (Gilbert et al., 2013, Qi et al., 2013), DNA methylation (McDonald et al., 2016, Vojta et al., 2016), and histone modifications (Hilton et al., 2015) have all been demonstrated. Activation or repression of
