The dCas9, also known as endonuclease deficient Cas9, is a mutant form of Cas9, whose endonuclease activity is removed by mutating the endonuclease domains. However?the dCas9 can still bind to its guide RNA and the DNA strand that is being targeted.
The dCas9 can be used in CRISPR interference (CRISPRi) as well as CRISPR activation (CRISPRa).
In CRISPRi, dCas9 binds to its DNA target but does not cleave it. The binding of Cas9 alone will prevent the cell’s transcription machinery from accessing the promoter, hence inhibiting the gene expression.
On the other hand, dCas9’s ability of binding target DNA can be exploited for activation, i.e. CRISPRa. A transcriptional activator is fused to dCas9, which can activate gene expression without changing DNA sequence.