Base Editing and Prime Editing
A plain-language overview of base editing and prime editing, how they differ from CRISPR cutting, the very early human evidence, and the open safety questions.
Content last updated . This page has not been medically reviewed.
Base editing
Base editing was reported in 2016 as a way to convert one DNA base into another in a programmable way, without a double-strand break or a donor template. In four human and mouse cell lines, the improved editors corrected about 15 to 75% of total cellular DNA with minimal indel formation, typically 1% or less. [1] That is a preclinical result in cell lines, not a treatment result.A 2026 review says base editing can in principle correct single-nucleotide variants, which make up more than half of known disease-causing variants, and lists early clinical success or promise in sickle cell disease, beta-thalassemia, high cholesterol, alpha-1-antitrypsin deficiency and glycogen storage disease type Ia. It also lists remaining challenges: bystander edits near the target, residual off-target DNA and RNA editing, delivery limits, immune reactions and the need for durable long-term safety evidence. [3]
Prime editing
Prime editing was described in 2019 as a "search-and-replace" method that writes new genetic information at a chosen site without double-strand breaks or donor DNA. In human cells the authors made more than 175 edits, and reported higher or similar efficiency and fewer byproducts than an older repair method, with much lower off-target editing than Cas9 nuclease at known sites. They estimated it could in principle correct up to 89% of known disease-associated variants. [2] That figure is an authors' estimate from cell work, not a patient outcome.A 2025 review says prime editing has corrected disease-causing changes in preclinical models, that the US FDA cleared an Investigational New Drug application for a first clinical trial (PM359, for chronic granulomatous disease, using cells edited outside the body), and that delivering the large components inside the body remains a major hurdle. [4]
Proper evidence not found for peer-reviewed human efficacy results from prime editing. I checked PubMed reviews and the FDA list of approved cellular and gene therapy products, read 2026-10-22. [5]
The first patient-specific example
In 2025 the New England Journal of Medicine reported a single infant with severe carbamoyl-phosphate synthetase 1 deficiency, a disease with an estimated 50% mortality in early infancy. A customized base-editing therapy delivered in lipid nanoparticles was developed for the child and, after regulatory approval, given in two infusions at about 7 and 8 months of age. Seven weeks after the first infusion the child could take more dietary protein and half the starting dose of a nitrogen-scavenger medication. No serious adverse events occurred, and the authors stated that longer follow-up is warranted to assess safety and efficacy. [6] This is one person, not a trial, and it does not show the approach works for others or for other conditions.Comparison at a glance
| Standard CRISPR-Cas9 | Base editing | Prime editing | |
|---|---|---|---|
| Double-strand break | Yes | No [1] | No [2] |
| Changes aimed at | Cuts, then repair | One base converted to another [1] | Many edit types, including small insertions and deletions [2] |
| Stage in this review | One approved product [7] | Early clinical reports [3] | IND cleared for a first trial [4] |
What to watch out for
- Hype. "Precise" does not mean risk-free. Reviews still list bystander and off-target edits and long-term safety as open questions. [3]
- Unproven clinics. A clinic offering "next-generation editing" does not establish approval. See Medical Tourism for Unproven Treatments.
- One case is not a trial. See Clinical-Trial Phases Explained.
Frequently asked questions
Is base editing approved anywhere?
No approved base-editing medicine was found in the sources read. The FDA list of approved cellular and gene therapy products was checked on 2026-10-22. [5] A 2026 review describes early clinical success rather than approval. [3]
What is the difference between base and prime editing?
Base editing converts one DNA base into another. [1] Prime editing writes new information at a site, including insertions and deletions. [2]
Does the infant case prove base editing works?
No. It was a single patient, and the authors say longer follow-up is needed. [6]
Is prime editing safer than CRISPR cutting?
In human cell experiments it caused much lower off-target editing than Cas9 nuclease at known sites. [2] Human safety data are limited, so no conclusion about patients is possible yet.
Are these treatments available to buy?
No. The one patient-specific case described above received treatment only after regulatory approval for that child. [6]
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
How to read the evidence labels
Labels used on this page: approved medicine, investigational, preclinical (cells or animals only) and insufficient evidence. See How We Label Evidence in Specialty Topics. For the background, read Gene Editing and CRISPR Explained.
Safety note
References
- Komor AC et al. Programmable editing of a target base in genomic DNA without double-stranded DNA cleavage. Nature 2016. Read 2026-10-22.
- Anzalone AV et al. Search-and-replace genome editing without double-strand breaks or donor DNA. Nature 2019. Read 2026-10-22.
- Base editing for precision therapeutics. Cell Genomics 2026. Read 2026-10-22.
- Aliciaslan M et al. Prime Editing: The Next Frontier in Precision Gene Therapy. J Gene Med 2025. Read 2026-10-22.
- US FDA, Approved Cellular and Gene Therapy Products. Read 2026-10-22.
- Musunuru K et al. Patient-Specific In Vivo Gene Editing to Treat a Rare Genetic Disease. N Engl J Med 2025. Read 2026-10-22.
- US FDA, FDA Approves First Gene Therapies to Treat Patients with Sickle Cell Disease (8 Dec 2023). Read 2026-10-22.