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categories: ["genetics", "foundations"]categories: ["genetics", "foundations"]tags: ["crispr", "genome editing", "molecular biology", "gene therapy", "nobel prize", "biotechnology"]tags: ["crispr", "genome editing", "molecular biology", "gene therapy", "nobel prize", "biotechnology"]summary: "A bacterial defence system repurposed as a programmable DNA-cutting tool, in which a short guide RNA directs the Cas9 enzyme to a chosen sequence in almost any genome."summary: "A bacterial defence system repurposed as a programmable DNA-cutting tool, in which a short guide RNA directs the Cas9 enzyme to a chosen sequence in almost any genome."updated: "2026-07-27"updated: "2026-10-29"humanEvidence: "One approved medicine, Casgevy, edits a patient's own blood stem cells outside the body; in vivo editing has durably lowered the disease-causing protein in transthyretin amyloidosis, and heritable editing is unapproved in every country with a relevant law."humanEvidence: "One approved medicine, Casgevy, edits a patient's own blood stem cells outside the body; in vivo editing has durably lowered the disease-causing protein in transthyretin amyloidosis, and heritable editing is unapproved in every country with a relevant law."access: "Reagents are ordinary catalogue items sold to any laboratory; the one approved medicine built on the platform treats two inherited blood disorders and is priced in the millions of dollars per patient."access: "Reagents are ordinary catalogue items sold to any laboratory; the one approved medicine built on the platform treats two inherited blood disorders and is priced in the millions of dollars per patient."reversibility: "irreversible"reversibility: "irreversible"lines 30–36 → 30–3615 unchanged lines not shown
}}`````` **CRISPR–Cas9** is a two-part molecular machine — a short guide RNA and a DNA-cutting protein — that bacteria use to destroy invading viral genomes and that biologists have repurposed to cut a chosen sequence in almost any organism. Its advantage over earlier editing platforms is that the target is specified by an interchangeable twenty-nucleotide RNA rather than by a redesigned protein, so retargeting takes days instead of months. Cutting, however, is only the first step. What an edit becomes depends on how the cell repairs the break, and that repair is the part biologists control least.**CRISPR-Cas9** is a two-part molecular machine (a short guide RNA and a DNA-cutting protein) that bacteria use to destroy invading viral genomes and that biologists have repurposed to cut a chosen sequence in almost any organism. Its advantage over earlier editing platforms is that the target is specified by an interchangeable twenty-nucleotide RNA rather than by a redesigned protein, so retargeting takes days instead of months. Cutting, however, is only the first step. What an edit becomes depends on how the cell repairs the break, and that repair is the part biologists control least. ```figure```figure{"key": "crispr-cas9-complex", "caption": "Cas9 holding a guide RNA against the unwound target strand. The enzyme does the cutting; the RNA decides where."}{"key": "crispr-cas9-complex", "caption": "Cas9 holding a guide RNA against the unwound target strand. The enzyme does the cutting; the RNA decides where."}lines 38–44 → 38–441 unchanged line not shown
## Origins in bacterial immunity## Origins in bacterial immunity The sequences now called CRISPR — clustered regularly interspaced short palindromic repeats — were first noticed in 1987 by Yoshizumi Ishino's group at Osaka University, who found an odd series of repeats downstream of a gene they were sequencing for unrelated reasons and could not explain them.[^ishino1987] Through the 1990s and early 2000s the same architecture turned up in many bacteria and archaea. Francisco Mojica at the University of Alicante catalogued the arrays, coined the acronym with Ruud Jansen, and proposed that the sequences between the repeats, called spacers, matched viral and plasmid DNA, implying an adaptive immune function.The sequences now called CRISPR (clustered regularly interspaced short palindromic repeats) were first noticed in 1987 by Yoshizumi Ishino's group at Osaka University, who found an odd series of repeats downstream of a gene they were sequencing for unrelated reasons and could not explain them.[^ishino1987] Through the 1990s and early 2000s the same architecture turned up in many bacteria and archaea. Francisco Mojica at the University of Alicante catalogued the arrays, coined the acronym with Ruud Jansen, and proposed that the sequences between the repeats, called spacers, matched viral and plasmid DNA, implying an adaptive immune function. ```timeline```timeline[[lines 58–68 → 58–6813 unchanged lines not shown
## How it works## How it works The system has two jobs that are worth separating, because they fail in different ways. The first is finding one twenty-base address in a genome of three billion. The second is doing something useful once it arrives. Cas9 performs the first job well and the second not at all: it makes a break and leaves the consequences to the cell.The system has two jobs, and they fail in different ways. The first is finding one twenty-base address in a genome of three billion. The second is doing something useful once it arrives. Cas9 performs the first job well and the second not at all: it makes a break and leaves the consequences to the cell. ### Targeting### Targeting The guide RNA carries a spacer of roughly twenty nucleotides complementary to the intended target. Cas9 does not scan the genome by reading that sequence directly. It first searches for a short motif adjacent to the target called the protospacer adjacent motif, or PAM — for the commonly used *Streptococcus pyogenes* enzyme, the three bases 5'-NGG-3'. Only after binding a PAM does Cas9 locally unwind the double helix and test whether the guide RNA can pair with the adjacent strand. A match holds the complex in place and triggers a conformational change that activates the enzyme; a mismatch lets it release and continue searching.The guide RNA carries a spacer of roughly twenty nucleotides complementary to the intended target. Cas9 does not scan the genome by reading that sequence directly. It first searches for a short motif adjacent to the target called the protospacer adjacent motif, or PAM. For the commonly used *Streptococcus pyogenes* enzyme that motif is the three bases 5'-NGG-3'. Only after binding a PAM does Cas9 locally unwind the double helix and test whether the guide RNA can pair with the adjacent strand. A match holds the complex in place and triggers a conformational change that activates the enzyme; a mismatch lets it release and continue searching. This two-stage check explains both the specificity and the limits of the system. The PAM requirement means not every site in a genome is addressable, though engineered variants with relaxed PAM preferences have widened the accessible fraction considerably. It also explains why mismatches distant from the PAM are tolerated more often than mismatches close to it, which is the root of most [[crispr-off-target-effects|off-target activity]].This two-stage check explains both the specificity and the limits of the system. The PAM requirement means not every site in a genome is addressable, though engineered variants with relaxed PAM preferences have widened the accessible fraction considerably. It also explains why mismatches distant from the PAM are tolerated more often than mismatches close to it, which is the root of most [[crispr-off-target-effects|off-target activity]]. lines 70–85 → 70–851 unchanged line not shown
Cas9 has two nuclease domains, HNH and RuvC, which cut the two DNA strands to leave a blunt double-strand break a few base pairs from the PAM. The cell then repairs it, and the repair pathway determines the outcome. Non-homologous end joining and related pathways rejoin the ends directly and frequently insert or delete a few bases, which shifts the reading frame and disables the gene. This is the easy, high-efficiency outcome, and it is why CRISPR is far better at breaking genes than at fixing them.Cas9 has two nuclease domains, HNH and RuvC, which cut the two DNA strands to leave a blunt double-strand break a few base pairs from the PAM. The cell then repairs it, and the repair pathway determines the outcome. Non-homologous end joining and related pathways rejoin the ends directly and frequently insert or delete a few bases, which shifts the reading frame and disables the gene. This is the easy, high-efficiency outcome, and it is why CRISPR is far better at breaking genes than at fixing them. Precise replacement requires homology-directed repair, which copies a supplied template. HDR operates mainly in dividing cells during S and G2 phase, competes poorly against end joining, and is inefficient in the post-mitotic cells that matter most in the nervous system, heart and muscle. Much of the field's subsequent engineering — [[base-editing]], [[prime-editing]] and [[epigenome-editing]] — exists to get useful changes without depending on a double-strand break at all.Precise replacement requires homology-directed repair, which copies a supplied template. HDR operates mainly in dividing cells during S and G2 phase, competes poorly against end joining, and is inefficient in the post-mitotic cells that matter most in the nervous system, heart and muscle. Much of the field's subsequent engineering ([[base-editing]], [[prime-editing]] and [[epigenome-editing]]) exists to get useful changes without depending on a double-strand break at all. > [!key] Why the break matters> [!key] Why the break matters> A double-strand break is a genuine chromosomal injury. The cell's response to it, not the nuclease, produces the edit — which is why the same guide RNA can give clean knockout in one cell type and a mixture of deletions, inversions and unedited alleles in another.> A double-strand break is a genuine chromosomal injury. The cell's response to it, rather than the nuclease, produces the edit. That is why the same guide RNA can give clean knockout in one cell type and a mixture of deletions, inversions and unedited alleles in another. ## Adaptation and the wider toolbox## Adaptation and the wider toolbox The Cas9 protein from *S. pyogenes* is about 1,360 amino acids, and its coding sequence sits uncomfortably close to the roughly 4.7 kilobase packaging limit of [[aav-vectors|adeno-associated viral vectors]]. Compact orthologues such as the *Staphylococcus aureus* enzyme were adopted partly for that reason. Other CRISPR effectors have distinct properties: Cas12a recognises T-rich PAMs and leaves staggered cuts, while Cas13 targets RNA rather than DNA and underpins several diagnostic assays. Sequence models have begun to generate Cas nuclease variants that were not found in any bacterium, an application of [[ai-protein-design]] to the editing toolkit itself. Catalytically dead Cas9, with both nuclease domains disabled, retains its RNA-programmed DNA binding and became the docking module for the transcriptional and epigenetic tools described elsewhere on this wiki.The Cas9 protein from *S. pyogenes* is about 1,360 amino acids, and its coding sequence sits uncomfortably close to the roughly 4.7 kilobase packaging limit of [[aav-vectors|adeno-associated viral vectors]]. Compact orthologues such as the *Staphylococcus aureus* enzyme were adopted partly for that reason. Other CRISPR effectors have distinct properties: Cas12a recognises T-rich PAMs and leaves staggered cuts, while Cas13 targets RNA rather than DNA and underpins several diagnostic assays. Sequence models have begun to generate Cas nuclease variants that were not found in any bacterium, an application of [[ai-protein-design]] to the editing toolkit itself. Catalytically dead Cas9, with both nuclease domains disabled, retains its RNA-programmed DNA binding and became the docking module for the transcriptional and epigenetic tools described elsewhere on this wiki. Delivery, not cutting, is the practical constraint. Ex vivo work usually electroporates a preassembled Cas9–guide ribonucleoprotein into cells outside the body, where dosing is controlled and the protein degrades within hours. In vivo work depends on [[lipid-nanoparticles]] or viral vectors, and inherits their tropism: nanoparticles reach hepatocytes easily and other tissues with difficulty.Delivery, not cutting, is the practical constraint. Ex vivo work usually electroporates a preassembled Cas9-guide ribonucleoprotein into cells outside the body, where dosing is controlled and the protein degrades within hours. In vivo work depends on [[lipid-nanoparticles]] or viral vectors, and inherits their tropism: nanoparticles reach hepatocytes easily and other tissues with difficulty. ## Current state## Current state lines 99–112 → 99–11213 unchanged lines not shown
The best-characterised hazards are genomic. Guides can cut at sequences resembling the target, and the resulting edits are unintended and permanent. On-target damage is arguably the larger problem: repair of a Cas9 break can produce kilobase-scale deletions, inversions and complex rearrangements that short-amplicon sequencing does not detect.[^kosicki2018] Breaks can also trigger a p53-mediated damage response, which selects against successfully edited cells and, in principle, for cells with impaired p53.The best-characterised hazards are genomic. Guides can cut at sequences resembling the target, and the resulting edits are unintended and permanent. On-target damage is arguably the larger problem: repair of a Cas9 break can produce kilobase-scale deletions, inversions and complex rearrangements that short-amplicon sequencing does not detect.[^kosicki2018] Breaks can also trigger a p53-mediated damage response, which selects against successfully edited cells and, in principle, for cells with impaired p53. The governance questions differ sharply between somatic and heritable use. [[somatic-gene-therapy]] affects only the treated person and is regulated as medicine. [[germline-editing|Heritable editing]] passes changes to descendants, cannot be consented to by the person affected, and after the [[he-jiankui-affair]] of 2018 is prohibited or unapproved in essentially every country with a relevant law — a situation examined in [[governance-of-genome-editing]]. The ease of the technique also makes it a standing example in debates over [[dual-use-research]], where the self-governance model of the [[asilomar-conference]] is repeatedly invoked and repeatedly disputed.The governance questions differ sharply between somatic and heritable use. [[somatic-gene-therapy]] affects only the treated person and is regulated as medicine. [[germline-editing|Heritable editing]] passes changes to descendants, cannot be consented to by the person affected, and after the [[he-jiankui-affair]] of 2018 is prohibited or unapproved in essentially every country with a relevant law, a situation examined in [[governance-of-genome-editing]]. The ease of the technique also makes it a standing example in debates over [[dual-use-research]], where the self-governance model of the [[asilomar-conference]] is repeatedly invoked and repeatedly disputed. > [!caution] Contested> [!caution] Contested> Claims that CRISPR is "precise" describe the targeting step, not the outcome. Guide binding is specific; the repair that follows is not, and characterising on-target rearrangements requires long-read or single-cell sequencing that many published studies did not perform.> Claims that CRISPR is "precise" describe the targeting step, not the outcome. Guide binding is specific; the repair that follows is not, and characterising on-target rearrangements requires long-read or single-cell sequencing that many published studies did not perform. ## The patent dispute## The patent dispute Ownership of the eukaryotic application has been litigated for more than a decade. The University of California, the University of Vienna and Charpentier filed first; the Broad Institute filed later and obtained early grants through accelerated examination. United States interference proceedings turned on who first conceived a working system in eukaryotic cells, and in 2022 the Patent Trial and Appeal Board awarded priority to the Broad. On appeal in 2025 the Federal Circuit vacated that decision and returned the case to the board, so the American position remains unsettled as of 2026. European outcomes have differed, partly on procedural grounds relating to priority claims. The 2020 Nobel Prize in Chemistry to Charpentier and Doudna, which omitted Feng Zhang and [[george-church]] among others, drew comment precisely because it did not track the patent fight.Ownership of the eukaryotic application has been litigated for more than a decade. The University of California, the University of Vienna and Charpentier filed first; the Broad Institute filed later and obtained early grants through accelerated examination. United States interference proceedings turned on who first conceived a working system in eukaryotic cells, and in 2022 the Patent Trial and Appeal Board awarded priority to the Broad. On appeal in 2025 the Federal Circuit vacated that decision and returned the case to the board, so the American position remains unsettled as of 2026. European outcomes have differed, partly on procedural grounds relating to priority claims. The 2020 Nobel Prize in Chemistry to Charpentier and Doudna, which omitted Feng Zhang and [[george-church]] among others, drew comment because it did not track the patent fight. ## Outlook## Outlook removed, struck through added, underlinedLine numbers count the serialised markdown of each revision, frontmatter included.
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