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How CRISPR Gene Editing Works
Genome editing is a group of technologies that let scientists add, remove or alter genetic material at particular locations in the genome. CRISPR-Cas9 is a well-known example, adapted from a system bacteria use as an immune defense.1 A small RNA with a short guide sequence binds to a target sequence in a cell’s DNA and to the Cas9 enzyme, and Cas9 cuts the DNA at that location. Researchers then use the cell’s own repair machinery to add or delete genetic material or replace a segment with a customized sequence.1 NHGRI describes CRISPR as a technology that research scientists use to selectively modify the DNA of living organisms.4
The Royal Swedish Academy of Sciences awarded the 2020 Nobel Prize in Chemistry to Emmanuelle Charpentier and Jennifer A. Doudna “for the development of a method for genome editing”.5 Most genome editing changes in people are limited to somatic cells and are not passed to the next generation; germline cell and embryo genome editing are currently illegal in the United States and many other countries, according to MedlinePlus Genetics.1
Which CRISPR Therapy Has FDA Approval?
On 8 December 2023 the FDA approved Casgevy for sickle cell disease in patients 12 years of age and older with recurrent vaso-occlusive crises, and described it as the first FDA-approved therapy using CRISPR/Cas9.2 Patients’ own blood stem cells are modified by genome editing, then returned in a one-time infusion that is part of a stem cell transplant. Before treatment, patients undergo myeloablative conditioning (high-dose chemotherapy), and the FDA said patients would be followed in a long-term study of safety and effectiveness.2 The FDA’s page listing approved cellular and gene therapy products, current as of 30 October 2026, includes Casgevy.6 Indications can change after approval, so check the current FDA label before relying on the ages or conditions stated here.
The table summarizes what the FDA documents report. Both studies are single-arm, so there was no comparison group.
Swipe sideways to see every column.
| Indication | FDA source | Treated | Evaluable | Primary outcome met |
|---|---|---|---|---|
| Sickle cell disease (12 years and older, recurrent vaso-occlusive crises) | News release, 8 Dec 2023 | 44 patients | 31 patients | 29 (93.5%): free of severe vaso-occlusive crises for at least 12 consecutive months during 24-month follow-up |
| Transfusion-dependent beta-thalassemia (12 years and older) | Summary Basis for Regulatory Action, 16 Jan 2024 (approval letter of the same date) | 52 subjects (interim analysis) | 35 subjects | 32 (91.4%): transfusion independence for at least 12 consecutive months, with weighted average hemoglobin of at least 9 g/dL and no transfusions |
Percentages in the last column were recalculated by Editverse from the counts the FDA reported.2,7 The most common side effects the FDA listed for Casgevy were low levels of platelets and white blood cells, mouth sores, nausea, musculoskeletal pain, abdominal pain, vomiting, febrile neutropenia, headache and itching.2 For the thalassemia study the FDA review gave a median follow-up of 23.6 months in the 35 evaluable subjects,7 so these figures describe early follow-up, not lifelong outcomes.
Approval status for the thalassemia indication: the FDA CASGEVY product page, current as of 2 July 2026, lists an approval letter dated 16 January 2024 under STN 125785 for patients aged 12 years and older with sickle cell disease with recurrent vaso-occlusive crises and transfusion-dependent beta-thalassemia.8 The same page lists a further application, STN 125787, with an indication for patients aged 2 years and older with either condition, so the ages in the table are those of the 2023 and 2024 documents; check the current label.8
What ClinicalTrials.gov Shows About CRISPR Trials: Editverse Analysis
To see where CRISPR research stands beyond one approved product, Editverse counted matching records in the public ClinicalTrials.gov registry. Only aggregate counts are reported here, and no record text is reproduced.
Method. On 31 October 2026 we queried the ClinicalTrials.gov API version 2 (data timestamp 30 October 2026) with the search string CRISPR OR Cas9 OR Cas12 OR "base editing" OR "prime editing", which returned 194 records.3 We kept records with study type Interventional and a stated phase (135 of 160; 25 interventional records had no phase, which the registry uses for studies such as device or behavioral trials, and 34 records were observational). Start year comes from the registered start date, which can be an anticipated date for studies not yet recruiting. We computed all counts in R and did not curate records by hand, so text matching may include records that are not gene-editing therapies; for example, a malaria challenge study with a genetically modified parasite is among the interventional records.
Result. Of the 135 interventional records with a stated phase, 120 (88.9%) were early phase 1, phase 1 or phase 1/2, and 15 (11.1%) were phase 2, phase 2/3 or phase 3; eight were phase 3.
Registered start years rose from 7 records in 2020 to 21 in 2025, and 23 records have a 2026 start date; 2026 is a partial year, so it includes planned starts.
Recruitment status of the interventional records with a stated phase (n = 135):
| Status | Records | Share |
|---|---|---|
| Recruiting | 43 | 31.9% |
| Not yet recruiting | 19 | 14.1% |
| Completed | 17 | 12.6% |
| Unknown status | 15 | 11.1% |
| Terminated | 14 | 10.4% |
| Active, not recruiting | 12 | 8.9% |
| Withdrawn | 8 | 5.9% |
| Enrolling by invitation | 6 | 4.4% |
| Suspended | 1 | 0.7% |
What this shows: on the query date, the registry held 135 interventional CRISPR-related records with a stated phase, most of them early phase, and the number of registered starts has grown since 2020. What it does not show: whether any of these treatments works, how many patients were treated, or how many trials are about gene-editing therapies alone. Sponsors register trials at different times, and statuses such as unknown are as submitted. Counts will change as the registry is updated.
Delivery and Off-Target Effects in CRISPR Therapy Research
A 2023 open-access review (CC BY 4.0) in Advanced Science describes the delivery route as a crucial aspect of clinical application and says plasmid and virus vectors have been used but their systemic effects, insertional mutagenesis and immune reactions have limited their use.9 The same review discusses polymer nanoparticles, liposomes and lipid nanoparticles as non-viral vectors under study.9 It also describes off-target effects as a significant concern that is an unavoidable issue for gene editing tools, with studies examining off-target rates and long-term side effects.9 For a manuscript, cite the delivery system and the method used to assess off-target activity for each product you discuss, as they differ between trials.
Using Registry and FDA Data in Your Own Review
- 1Cite the primary document. Use the FDA news release, review memo or label for approval dates, indications and trial numbers rather than a news summary.
- 2Record the query. Report the search string, the date you ran it, the data timestamp and the API version, because the registry changes daily.
- 3Define your filters. State study type, phase and date rules, and how you treated missing phases and anticipated start dates.
- 4Separate counts from evidence. Describe registry counts as records and use published trial reports for efficacy or safety.
Need the Analysis Run or Checked? Consult Editverse
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Frequently Asked Questions
What is CRISPR gene editing?
CRISPR is short for clustered regularly interspaced short palindromic repeats. It was adapted from a natural genome editing system that bacteria use as an immune defense. Genome editing technologies let scientists add, remove or alter genetic material at particular locations in the genome.
How does CRISPR-Cas9 work?
A small piece of RNA with a short guide sequence binds to a target sequence in a cell and also attaches to the Cas9 enzyme. Cas9 then cuts the DNA at that location, and researchers use the repair machinery already in the cell to add or delete genetic material or replace a segment with a customized sequence.
Which CRISPR therapy has FDA approval?
Casgevy (exagamglogene autotemcel) was approved by the FDA on 8 December 2023 for sickle cell disease in patients 12 years of age and older with recurrent vaso-occlusive crises, and the FDA product page lists an approval letter dated 16 January 2024 for its transfusion-dependent beta-thalassemia indication. The same page, current as of 2 July 2026, also lists STN 125787 for patients aged 2 years and older. Indications can change, so check the current FDA label.
What did the FDA documents report for Casgevy?
In the sickle cell disease trial, 44 patients were treated and 29 of 31 evaluable patients (93.5%) had no severe vaso-occlusive crises for at least 12 consecutive months. In the thalassemia study, 52 subjects were dosed and 32 of 35 evaluable subjects (91.4%) reached transfusion independence. Both are single-arm trials, and the thalassemia figures are from an interim analysis.
How many CRISPR-related trials are registered on ClinicalTrials.gov?
In an Editverse analysis run on 31 October 2026, 194 records matched CRISPR, Cas9, Cas12, base editing or prime editing in the text. Of these, 160 were interventional and 135 of those had a stated phase. Text matching can include records that are not gene-editing therapies, and a registry record does not show that a treatment works.
Are most CRISPR trials late stage?
No. Of the 135 interventional records with a stated phase, 120 (88.9%) were early phase 1, phase 1 or phase 1/2, and 15 (11.1%) were phase 2, phase 2/3 or phase 3.
Can CRISPR change genes that are passed to children?
MedlinePlus Genetics states that most genome editing changes are limited to somatic cells and are not passed to the next generation. Changes made to egg or sperm cells or to an embryo could be passed on, and it states that germline cell and embryo genome editing are currently illegal in the United States and many other countries.
How should I report a ClinicalTrials.gov analysis in a manuscript?
State the exact search string, the date you ran the query, the data timestamp, the API version, which fields you used and how you handled records with a missing phase or start date. Cite ClinicalTrials.gov as the source and report counts as records, not as proof of efficacy.
References
- MedlinePlus Genetics, National Library of Medicine. What are genome editing and CRISPR-Cas9? medlineplus.gov/genetics/understanding/genomicresearch/genomeediting/
- U.S. Food and Drug Administration. FDA Approves First Gene Therapies to Treat Patients with Sickle Cell Disease (news release, 8 December 2023). www.fda.gov/news-events/press-announcements/fda-approves-first-gene-therapies-treat-patients-sickle-cell-disease
- ClinicalTrials.gov, National Library of Medicine. ClinicalTrials.gov API version 2 (data timestamp 30 October 2026; queried by Editverse on 31 October 2026). clinicaltrials.gov/data-api/api
- National Human Genome Research Institute. Talking Glossary of Genomic and Genetic Terms: CRISPR. www.genome.gov/genetics-glossary/CRISPR
- The Royal Swedish Academy of Sciences. Press release: The Nobel Prize in Chemistry 2020 (7 October 2020). www.nobelprize.org/prizes/chemistry/2020/press-release/
- U.S. Food and Drug Administration. Approved Cellular and Gene Therapy Products (page content current as of 30 October 2026). www.fda.gov/vaccines-blood-biologics/cellular-gene-therapy-products/approved-cellular-and-gene-therapy-products
- U.S. Food and Drug Administration, CBER. Summary Basis for Regulatory Action: CASGEVY (exagamglogene autotemcel), BLA STN 125785, transfusion-dependent beta-thalassemia (16 January 2024). www.fda.gov/media/175842/download
- U.S. Food and Drug Administration, CBER. CASGEVY: product page with indications, approval letters and review documents for STN 125785 and STN 125787 (page content current as of 2 July 2026). www.fda.gov/vaccines-blood-biologics/casgevy
- Zheng R, Zhang L, Parvin R, Su L, Chi J, Shi K, Ye F, Huang X. Progress and perspective of CRISPR-Cas9 technology in translational medicine. Adv Sci (Weinh). 2023;10(25):e2300195. doi:10.1002/advs.202300195. CC BY 4.0, PMC10477906. pmc.ncbi.nlm.nih.gov/articles/PMC10477906/
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Editverse Research Team | Updated October 2026