Fig. 3

Applications and targets of CRISPR-Cas systems. CRISPR-Cas systems can target various types of nucleic acids, including invasive and mobile DNA (green box), or endogenous sequences (blue box). In their native environment, CRISPR-Cas systems naturally target mobile and exogenous DNA elements. Conversely, engineered systems are typically designed to target self-DNA to trigger endogenous modifications. Targeting can be directed at bacteriophage DNA to provide anti-viral defense (upper left). Likewise, Cas nucleases can be directed at plasmid DNA in order to prevent uptake and dissemination of undesirable sequences or to cure the host of plasmid sequences (center left). Targeting can also be directed at mobile DNA elements such as transposons so as to maintain DNA integrity and ensure homeostasis (lower left). When aiming the CRISPR-Cas machinery towards the cell’s own chromosomal content, the purpose is typically to induce endogenous DNA repair pathways to drive editing of the DNA sequence (upper center). Catalytically deactivated variants of Cas nucleases can be used as DNA-binding proteins to block transcription (CRISPRi, upper right), or can be fused to transcriptional activators to activate transcription (CRISPRa, center right). Alternatively, Cas nucleases can be reprogrammed to trigger a lethal auto-immune response, leading to cell death (bottom right). CRISPR sequences themselves can be used for genotyping, by using the series of vaccination events as a genetic historical record (lower center). Cas CRISPR associated, CRISPR clustered regularly interspaced short palindromic repeat, CRISPRa CRISPR activation, CRISPRi CRISPR interference