CRISPR interference (CRISPRi) is a genetic perturbation technique that inhibits gene expression by targeting a nuclease dead version of Cas9 (dCas9) to a region near the transcription start site (TSS). Thereby, the cell’s transcription machinery is prevented from accessing the TSS, resulting in the inhibition of gene expression.

CRISPR activation (CRISPRa) is also a type of CRISPR tool that employs the modified versions of dCas9. In contrast to CRISPRi, transcriptional activators are added on dCas9 or the guide RNAs, aiming to increase, rather than inhibit, expression of genes of interest.