modified guide RNA scaffolds then allow recruitment of specific enzymatic activities to desired sites in the DNA (Dominguez et al. 2016; Konermann et al. 2013) (Fig. 4g). This has been used extensively to manipulate gene expression both positively using transcriptional activation domains such as VP16, Rta, p65 or HSF1 and negatively by fusion to KRAB, and genome-wide screens using these reagents have been successfully implemented (Gilbert et al. 2014; Konermann et al. 2015). Such systems have been used to for instance manipulate alpha-synuclein levels both positively and negatively in neurons derived from patients with a triplication of the SNCA locus, allowing manipulation of SNCA levels in this disease model (Heman-Ackah et al. 2016). Equally importantly, a variety of chromatin modifying activities can also be recruited, including the p300 histone acetyltransferase (Hilton et al. 2015) or LSD1 histone demethylase (Kearns et al. 2015) to activate or inactivate enhancers, the Dnmt3 DNA methyltransferases (Amabile et al. 2016; Liu et al. 2016a, b; Vojta et al. 2016) or TET1 demethylase (Choudhury et al. 2016; Liu et al. 2016a, b; Xu et al. 2016) to add or remove DNA methylation marks, or other chromatin modifying enzymes such as G9a or SUV39H1 (Snowden et al.
