The dCas9(D10A.H840A) entry in FlyBase represents sequence encoding a nuclease-dead form of the Streptococcus pyogenes Cas9 gene that contains the D10A mutation in the RuvC domain and the H840A mutation in the HNH domain. Nuclear-targeted dCas9(D10A.H840A) can be used on its own for transcription interference (CRISPRi); when expressed in combination with a single guide RNA (sgRNA) that targets it to a promoter, the promoter-bound dCas9(D10A.H840A) can sterically interfere with the formation of the pre-initiation complex, disrupting transcription initiation. Alternatively, if dCas9(D10A.H840A) is targeted to the nontemplate DNA strand of a UTR or coding region, it can block transcription elongation (PMID:24136345). If a protein of interest is tagged with dCas9(D10A.H840A), then that protein can be 'targeted' to a particular site in the genome by combining it with an appropriate sgRNA; this property can be used to study the effect of the tagged protein of interest on chromosome organization and/or gene expression.