Several groups have successfully applied the CRISPR-dCas9 system to modulate gene expression levels for large-scale genome-wide screens (Carlson-Stevermer et al., 2016, Gilbert et al., 2014, Horlbeck et al., 2016a, Konermann et al., 2015). Our findings do not suggest that these dCas9 platforms are unsuitable for genome-wide approaches; rather, we only suggest that when focusing on a small number of candidate genes, it is prudent to always empirically establish the magnitude of gene expression modulation achieved. The successes of these CRISPR screens could reflect technical differences between our approaches (such as the levels of dCas9/gRNA expression, timing of dCas9 and/or gRNA selection, or the biology of specific cell lines used). We do not believe that the variability in gRNA efficacy reflects vector integration effects (such as differences in copy number or expression levels of dCas9 and/or gRNAs), as our FACS (Figure S2A) and qPCR (Figures S2B and S2C) data suggest that the difference in dCas9 expression across individuals is small, an observation that is consistent with findings that dCas9−KRAB expression does not correlate to the level of changes gene expression achieved
