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 transcription using dCas9-fusion protein variants represents a novel methodology to design gain- or loss-of-function studies with high fidelity. As this modulation occurs at the endogenous level, it is predicted to include the full range of alternative splice isoforms, which are frequently overlooked by RNAi technologies or the use of cDNA overexpression approaches. While a growing number of proof-of-concept studies have demonstrated the successful application of a variety of dCas9-fusion proteins to the up- and downregulation of endogenous expression, few, if any, have systematically described the inter-gene, inter-individual, inter-cell type, and inter-effector variation in the practical application of this system.
