Controlling CRISPR: a review of current anti-CRISPR technologies

Anti-CRISPR systems are emerging as essential regulators of CRISPR-Cas technologies, providing a controllable “off-switch” for genome editing, transcriptional regulation, and gene-drive applications. This review surveys the expanding landscape of anti-CRISPR strategies, including naturally occurring Acr proteins, engineered Acr switches, small-molecule inhibitors, and nucleic acid-based inhibitors. Acr proteins, discovered in phages and other mobile genetic elements, exploit diverse mechanisms such as blocking CRISPR-Cas complex assembly, preventing target DNA or RNA binding, or directly inhibiting catalysis. Recent work has revealed additional noncanonical mechanisms, including RNA mimicry, host-factor hijacking, and enzymatic modifications of Cas proteins. In parallel, computational screening and machine-learning methods have accelerated discovery of Acr families and enabled de novo design of Cas13 inhibitors. This review also examines engineered control systems that make Acr activity conditional on chemical, light, miRNA, protease, or other environmental cues, enabling spatial and temporal regulation of CRISPR function. Small-molecule inhibitors and nucleic-acid inhibitors offer complementary CRISPR control, including the potential for improved delivery, tunable dosing, and modular design, though further assessment in therapeutic applications is necessary. Across applications, anti-CRISPRs have shown utility in improving editing precision, reducing toxicity, constraining gene drives, controlling CRISPRi/CRISPRa systems, and enhancing safety in therapeutic and agricultural contexts. Despite substantial progress, important challenges remain, including delivery, specificity across natural and engineered CRISPR variants, and governance. Future efforts will likely focus on rational design, multiplexed inhibition, and integration of anti-CRISPR systems into existing CRISPR-Cas therapeutic and environmental frameworks. As CRISPR continues to expand beyond the laboratory into medicine, agriculture, and ecological engineering, anti-CRISPR technologies will play a central role in enabling safer and more precise genome engineering.