restore Cas9 access (Horlbeck et al., 2016b), it is possible that chromatin-modifying enzymes may improve efficacy, although the impact of such treatment on disease-modeling experiments is unknown. It is also possible that alternative and/or combinations of dCas9 effectors acting further from the promoter (Hilton et al., 2015) or that directly modify DNA (Liu et al., 2016) or histone (Kearns et al., 2014) methylation may improve gRNA reliability across genes, individuals, and cell types. Finally, although not tested here, given the reported epigenetic differences between hiPSCs generated from the same individual (Lister et al., 2011, Ma et al., 2014, Mekhoubad et al., 2012, Nazor et al., 2012, Ruiz et al., 2012), it will be interesting to test to what extent gRNA efficacies can vary across isogenic NPCs, neurons, or astrocyte populations differentiated from independent hiPSCs from the same individual; of course, sequencing the promoter regions of each individual at each target gene would be necessary to rule out genetic effects.
