CRISPR-based tools have quickly moved from specialist techniques to routine instruments in biology and medicine, and they are now central to large-scale loss-of-function and perturbation screens. In this review, we focus on how pooled CRISPR screens are used to interrogate gene function in living cells, most often through cell fitness or simple selectable markers, and contrast this with arrayed formats that trade throughput for richer molecular readouts, such as transcriptome-wide changes. We bring together current strategies for library design, delivery, and selection and show how different Cas nucleases, including Cas9, Cas12, and Cas13, broaden the range of genome and transcriptome perturbations that can be assayed. We then discuss recent applications in drug response, viral infection, and cancer biology and consider how improvements in high-content technologies, data analysis, and emerging diagnostic uses are likely to shape the next generation of CRISPR-based screening studies.

CRISPR Cas9 revolutionizing genetic engineering and therapeutic applications.

The Cas9 nuclease has become central to modern methods and technologies in synthetic biology, largely due to the ease with which it can be targeted to specific DNA loci via guide RNAs (gRNAs). Reports vary widely on the actual specificity of this targeting, with some studies observing 60% of gRNAs possessing no activity against the genome, yet an assumption persists within the E. coli community that inactive gRNAs are rare. To resolve these contradictions, we evaluated the activity of 463 000 unique gRNAs in the E. coli K12 MG1655 genome. We show that the overwhelming majority (at least 93%) of unique gRNAs are functional while only 0.3% are nonfunctional. These nonfunctional gRNAs exhibit strong spacer self-interaction, which can either be excluded using a simple design rule or "repaired" during library design. Finally, this work provides the greater microbial synthetic biology community both a set of nearly half a million empirically evaluated E. coli gRNAs as well as a thoroughly evaluated experimental procedure, complete with appropriate controls for Cas9 activity, for conducting Cas9 assays in E. coli specifically and bacteria more generally. Lastly, we have produced a webapp to allow users to easily browse and extract gRNA sequences from the E. coli genome, which can be accessed at https://grna.ornl.gov.

Streptomycetes are prolific producers of bioactive natural products, but many of the biosynthetic gene clusters (BGCs) are silent in the laboratory. Genetic manipulation is important to unlock their full potential. CRISPR–Cas-based genome editing has greatly advanced genetic engineering in Streptomyces. However, several challenges remain, including Cas nuclease toxicity, unintended genomic rearrangements, and elimination of the delivery plasmid. Here, we present a novel genome editing strategy that harnesses cumate-inducible CRISPR interference (CRISPRi) to transiently knockdown essential genes such as divIVA or dnaA as counterselectable marker. This enforces loss of the vector backbone, promotes homologous recombination, and yields markerless mutants by loss of the antibiotic resistance cassette during the final recombination step. We demonstrate the versatility of the ICE system (Inducible CRISPRi targeting an Essential gene) by (i) deleting four BGCs in Streptomyces coelicolor M145, (ii) inserting both a promoter and a large BGC, and (iii) introducing precise single-nucleotide substitutions. Furthermore, deletion of the prodigiosin BGC elicited expression of a poorly expressed BGC for prolinolexin lipopeptides in Streptomyces roseifaciens DSM 106196T. Considering that different essential genes may be targeted, we anticipate that inducible CRISPRi-based counterselection may be adaptable to genome editing strategies in a broad range of microbial systems.

Angelman syndrome (AS) is a severe neurodevelopmental disorder caused by loss of maternal UBE3A. In neurons, the paternal (pat)UBE3A allele is silenced by a long non-coding antisense transcript called Ube3a-ATS. Previous genome-editing approaches used active nucleases to unsilence patUbe3a by disrupting Ube3a-ATS. However, these methods create DNA double-strand breaks (DSBs) and promote integration of adeno-associated virus (AAV) vector genomes, both of which raise potential safety concerns. Here, we found that a nickase Neisseria meningitidis Cas9 variant (nNmCas9-D15A) disrupted Ube3a-ATS transcription when targeted to the non-template strand and unsilenced patUbe3a in cultured mouse neurons without generating DSBs or causing AAV integration. Intracerebroventricular delivery of AAV9-nNmCas9-D15A in AS model mice potently and durably reduced Ube3a-ATS and elevated Ube3a throughout the cerebral cortex and hippocampus for at least 6 months. Further, this vector restored UBE3A expression in ~ 87% of cortical neurons, which compares favorably to previously reported efficiencies with active Cas9, dead Cas9, and zinc finger nuclease vectors. These results demonstrate that nNmCas9 is a highly effective and potentially safer genome editor for the treatment of AS.

Genome editing can enhance basic research and enable industrial applications of green algae. Here, we present an affordable, broadly applicable workflow for genome editing in the unicellular green alga Euglena gracilis using Cas9 nucleases. This method retains high editing efficiency while significantly lowering technical barriers. Unlike previous approaches that required specialized equipment, this protocol can be performed using a general-purpose laboratory electroporator and a simplified clonal isolation procedure without the need for specialized micromanipulation devices. This protocol is compatible with a range of editing outcomes, such as targeted deletions and precise base substitutions, enabling more widespread genome editing in Euglena. © 2026 The Author(s). Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Culture of Euglena gracilis Basic Protocol 2: sgRNA synthesis Basic protocol 3: Transformation Basic protocol 4: Genotyping.

Therapeutic gene editing with designer nucleases can be compromised by undesired repair outcomes. DNA repair inhibitors are used to bias DSB repair toward HDR, but their impact on larger structural rearrangements, including large deletions and translocations, remains unclear. We quantify the mutational burden associated with end-joining inhibitor compounds. With a highly precise Cas9 nuclease, repair inhibition yields modest increases in aberrations, whereas promiscuous single guide RNAs (sgRNAs) amplify aberrant outcomes by orders of magnitude. Donor templates mitigate mutational burden at on-target sites, and in rare cases donor sequences bridge translocations between on- and off-target loci. Because DNA-PKcs inhibition does not itself induce instability over short intervals but increases the likelihood of capturing chromosomal aberrations postediting, we leveraged this to enhance assay performance. Compared to CAST-Seq, high-resolution CAST-Seq achieved a median ~12-fold increase in detected aberrations and, in this higher-sensitivity context, revealed unintended, target-specific sgRNA contaminants in GMP-like batches, underscoring direct genotoxicity risk and the need for stricter guide purity controls. A modified, translocation-quantitative rhAmpSeq reports all translocation combinations between two loci, enabling robust off-target validation beyond indel-only readouts. Finally, we evaluate AZD7648, finding limited aberration increases with precise nucleases and reconciling reports of extensive large deletions by quantifying assay- and design-dependent biases.

Engineering proteins with desired functions remains challenging and usually requires multiple rounds of screening and selection. Here, we present Sequence Display, a platform that generates large-scale protein sequence–activity datasets in a single round. Sequence Display enables multiplexed assessment of individual variant activity within a single experiment, offering a robust approach to mapping detailed sequence–function relationships. We demonstrate the platform’s broad applicability by generating datasets for cytosine deaminase, uracil glycosylase inhibitor, aminoacyl-tRNA synthetase, and a compact Cas9 nuclease. Integrating these datasets obtained from Sequence Display with pre-trained protein language models, fine-grained, variant-specific activity landscapes can be constructed. We discovered several Cas9 variants with expanded protospacer adjacent motif and evolved aminoacyl-tRNA synthetase variants capable of recognizing different non-canonical amino acids. Together, this study establishes Sequence Display as a powerful tool for mapping protein activity landscapes and accelerating the discovery of optimized proteins for biological and medical applications.

CRISPR-based correction of apolipoprotein E4 in Alzheimer's disease: Therapeutic strategies and macromolecular delivery innovations.

Here we identify a novel therapeutic opportunity for over 500 genes with putative “dominant & dispensable” (D&D) disease alleles. In these haplosufficientgenes, a single functional allele may be sufficient for health, presenting the opportunity for therapeutic approaches that silence the pathogenic allele. We show that allele-specific targeting of common heterozygous genetic variation linked to D&D alleles enables a disease mutation-agnostic gene therapy approach that increases the number of patients treatable with a single therapy. In some disease genes, this approach would allow >80 times as many patients to be treated as mutation-specific strategies. D&D alleles cause diverse diseases, including neurodegeneration, cardiomyopathies, retinopathies, and diabetes, demonstrating the therapeutic opportunity of this approach across physiological systems. To enable broad application of allele-specific mutation-agnostic targeting, we provide genome-wide maps of common heterozygous variants that support D&D disease allele disruption by multiple CRISPR-based editing technologies, including Cas9 nucleases, base editors, and epigenome editors.

Current one-pot clustered regularly interspaced short palindromic repeats diagnostics are limited by the cis-cleavage activity of Cas nucleases, which leads to amplicon degradation during amplification. Here, we report a streamlined strategy that overcomes this limitation. By integrating a bipartite split-crRNA into Cas12a (SCas12a), we separate target recognition from PAM dependency and completely eliminate cis-cleavage while preserving robust trans-cleavage. This strategy is broadly applicable for one-pot testing, compatible with recombinase polymerase amplification, RT-RPA, and loop-mediated isothermal amplification, as well as multiple Cas12a orthologs, including As, Lb, and Ct Cas12a. Moreover, the SCas12a accelerates one-pot testing with 100-1000-fold improved sensitivity and achieves >10-fold reduction in time-to-signal, enabling detection of targets at attomolar levels within 30 min. Additionally, it provides single-base resolution with up to 91-fold selectivity. The system has been successfully applied to detect HPV16, SARS-CoV-2, and TP53 SNPs in clinical samples. Together, we have developed a PAM-independent and cis-cleavage-free one-pot Cas12a assay, which holds strong potential for point-of-care diagnostics.

Casδ is a recently identified evolutionary transitional CRISPR system characterized by its compact size (~900 amino acids), broad temperature tolerance, and guidance with a short crRNA without the requirement of a tracrRNA. However, the low editing efficiency of Casδ in eukaryotic cells limits its application. Here, we have developed a hierarchical engineering strategy to improve the genome editing activity of Casδ-1, with optimization focused on enhancing its interactions with the crRNA, the protospacer adjacent motif (PAM) duplex, the single-stranded DNA substrate, and the RNA-DNA heteroduplex. Through this strategy, we successfully generated an activity-enhanced Casδ-1 variant, designated enCasδ, which harbors 9 amino acid substitutions that synergistically augment its editing efficiency. In human cell lines, enCasδ showed 1.3- to 29.3-fold higher editing activity than the wild-type Casδ-1 across ten tested genomic loci, with an average editing efficiency of 54.6%. In addition, enCasδ also mediated robust genome editing in maize; its editing efficiency increased by an average of 5.3-fold relative to Casδ-1, and reached up to an average of 80% at the TS4 and PSY1 loci in stable transgenic lines. The overall editing performance of enCasδ was comparable to that of Streptococcus pyogenes Cas9 (SpCas9) and other Cas12 nucleases. Collectively, enCasδ represents a highly optimized Casδ-1 variant that broadens the applicability of the Casδ CRISPR system and facilitates robust genome editing in both animal cells and plants.

Hypoxic tumors overexpress nitroreductase (NTR), providing an endogenous trigger for selective biomolecular activation. Here, we describe the synthesis of NTR-responsive clustered regularly interspaced short palindromic repeats (CRISPR) guide RNAs via the site-specific incorporation of a p-nitrobenzyl (p-NB) phosphoramidite at the 5' terminus of crRNAs. Click-mediated oligomerization into trimeric and tetrameric constructs effectively suppressed Cas nuclease activity. Enzymatic reduction by NTR induced linker cleavage, releasing active crRNAs and restoring DNA cleavage in vitro, establishing a strategy for enzyme-regulated CRISPR control.

Structural insights into Cas9-mediated prespacer selection in CRISPR-Cas adaptation.

Adenine base editor for knockout of proteins: A practical guide from design to analysis with updated MultiEditRbatch.

Robust biocontainment is essential for the safe use of engineered microbes, but existing strategies suffer from genetic instability and/or laborious construction. Here, we present ATTACH, a kill switch that associates toxin-antitoxin with CRISPR-Cas to harness engineered microbes. Our approach employs a CRISPR-repressed toxin-antitoxin (CreTA) module to make microbes addicted to the type I-F Cas effector proteins, and places both the Cas3 nuclease and the chromosome-targeting guide RNA under inducible promoters, thereby improving the genetic stability and stringency of the CRISPR-based suicidal program. Additionally, we have developed a single-plasmid, antibiotic-independent ATTACH device, which shows robust, stringent containment of a microbial chassis in murine gut, and negligible impacts on culture growth or lycopene production during batch fermentation. Our data highlight the potential of CreTA to stabilize CRISPR-based kill switches, advancing their development into more portable and reliable biocontainment tools for engineered microbes.

Currently, there is no permanent treatment for the group of severe monogenic fragile skin conditions epidermolysis bullosa (EB). The recent USFood and Drug Administration (FDA)-approved in vivo gene replacement therapy beremagene geperpavec (Vyjuvek®) provides a promising solution, but it requires ongoing application and is not applicable to all forms of EB. Targeted gene editing approaches directly addressing pathogenic mutations hold great promise for the development of durable personalized therapies. Here, we comprehensively describe the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) gene editing landscape for EB, critically review the advantages and limitations of emerging therapeutic strategies, and present some future perspectives. We find that the widespread application of Cas9 nuclease is currently hindered by off-target genotoxicity, which can be mitigated using Cas9 nickases. Further, new tools including prime editing have recently emerged and hold significant potential for EB gene therapy. Ongoing developments in gene editing technologies focused on improving safety and editing precision offer significant promise for the future clinical translation of potentially lifelong treatments for people with EB.

ABSTRACTChlamydomonas reinhardtii is a model green microalga that has great industrial potential as a sustainable bio‐factory for recombinant protein and high‐value chemical production. Efficient genome editing tools are required to redesign this organism for synthetic biology applications. CRISPR‐Cas editing technologies have already been adapted for gene knockout, transgene knock‐in, and precise gene editing in C. reinhardtii. However, the efficacy of CRISPR/Cas‐mediated gene knockout (KO) is low, which hampers pathway engineering and functional genomic studies. Here we report that co‐delivery of CRISPR‐Cas gene editing reagents with short double‐stranded non‐homologous oligodeoxynucleotides (dsNHO) increases gene knockout efficacy up to 100‐fold in C. reinhardtii. This phenomenon, referred to as non‐homologous oligonucleotide enhancement (NOE), is heavily affected by the length, structure, and chemical modifications of dsNHO, and is largely mediated by the DNA double‐stranded break sensor KU70/80 (KU) heterodimer in a Cas nuclease‐, locus‐, and strain‐independent manner. Our data suggest that dsNHOs disrupt the cell's double‐stranded break (DSB) sensing pathways, consequently shifting the balance of DNA repair from canonical non‐homologous end joining (c‐NHEJ) towards the more error‐prone, microhomology‐mediated end joining (MMEJ), which could be harnessed as a strategy for improving gene KO efficiency in Chlamydomonas and beyond.

Protein structure prediction using deep learning has revolutionized protein design. Yet, our understanding of protein function remains a key limitation for designing novel proteins that perform complex biological tasks. Here, we adopt a massively-parallel, function-first approach to rationally design synthetic proteins. Using genome-scale CRISPR activation, we overexpress ∼19,000 human proteins and measure their impact on precise gene editing. We identify over 800 native proteins that promote homology-directed repair. Using top candidates, we then design synthetic genome editors - Targeted Repair fUsion Editors (TruEditors) - by fusing full-length proteins or smaller core domains to the Cas9 nuclease. We develop 12 unique TruEditors that improve precise gene editing in diverse cell types and at genomic loci where existing methods for precise gene editing fail. Using affinity proteomics, we show that these synthetic proteins work by coordinating with endogenous DNA repair complexes. The delivery of TruEditors via mRNA more than doubles the rate of chimeric antigen receptor (CAR) insertion into the TRAC locus of primary human T cells, enhancing CAR T cell-directed tumor cell killing, and improves precise editing in human pluripotent stem cells more than three-fold. Overall, our study demonstrates that genome-wide protein overexpression screens can guide the rational design of synthetic proteins for specific biological tasks.

CRISPR–Cas9 nucleases have transformed genome engineering, yet their application is often constrained by protospacer-adjacent motif (PAM) requirements. Staphylococcus aureus Cas9 (SaCas9) is particularly attractive for in vivo applications due to its compact size; however, its NNGRRT PAM limits targetable genomic sites. Here, we report KRH, a SaCas9 variant designed entirely from the wild-type enzyme through a fully computational point-mutation design workflow, UniDesign, without additional experimental optimization. As expected, KRH efficiently recognizes an expanded NNNRRT PAM and exhibits substantially enhanced editing efficiency at non-canonical PAM sites, with improvements of up to 116-fold over the wild type. Across multiple human cell types, KRH achieves genome- and base-editing efficiencies comparable to, or exceeding, those of the well-known evolution-derived KKH variant. Computational modeling by UniDesign provides a mechanistic explanation for the PAM relaxation observed in both KRH and KKH, with structural and energetic analyses revealing that KRH relaxes PAM specificity by fine-tuning the balance between sequence-specific interactions with PAM bases and nonspecific contacts with the DNA backbone. Beyond its practical utility, KRH demonstrates that computational design can identify a minimal set of mutations sufficient to remodel the PAM interface while preserving high nuclease activity. This approach recapitulates—and in some cases surpasses—the performance of evolution-derived variants, offering a scalable strategy for high-throughput Cas9 engineering. Overall, these results establish KRH as a blueprint for rationally engineered, PAM-relaxed nucleases and underscores the power of computational design to accelerate next-generation genome editing.