Base editing

Base editing converts one DNA letter directly into another at a chosen position, using an enzyme that chemically modifies a base rather than a nuclease that severs the chromosome. It was built to solve a specific weakness of CRISPR–Cas9: most disease-causing mutations are single-letter substitutions, and correcting a single letter with a double-strand break is inefficient and messy. Base editors handle four of the twelve possible substitutions, which covers a large fraction of known pathogenic point mutations but by no means all of them.

§How it works

A base editor is a fusion protein. The targeting half is a catalytically impaired Cas9 that binds DNA under guide-RNA control but cuts at most one strand. The working half is a deaminase, an enzyme that strips an amine group from a base and changes what the cell's polymerases read.

Cytosine base editors, described in 2016, use a cytidine deaminase to convert cytosine to uracil.¹ Uracil is read as thymine during replication, so a C•G pair becomes T•A. Because cells actively excise uracil from DNA, the editor also carries a uracil glycosylase inhibitor to block that repair, and nicks the opposite strand so the cell rebuilds it using the edited strand as template.

Adenine base editors, described in 2017, had no natural starting enzyme: no known deaminase acts on adenine in DNA. The solution was directed evolution of a bacterial tRNA adenine deaminase until it accepted single-stranded DNA.² The product, inosine, is read as guanine, converting A•T to G•C.

Both editors act on the short stretch of DNA displaced when Cas9 unwinds the helix, giving an editing window of roughly five nucleotides within the protospacer. The window is the source of the method's characteristic error: if more than one editable base falls inside it, all of them may be converted. These bystander edits are usually silent or tolerated, but they are not always, and guide design often becomes a search for a position that puts only the intended base in range.

§Development history

Both classes came out of David Liu's laboratory, and both were engineered rather than discovered: the adenine editor in particular required many rounds of laboratory evolution before it worked on DNA at all. Successive generations improved on-target activity, narrowed the editing window, and reduced the deaminase's promiscuity. Faster variants shortened the exposure time needed, which matters because most unintended editing accumulates with duration of expression.

The first use in a person came in 2022, when a team at Great Ormond Street Hospital in London treated a teenager with relapsed T-cell acute lymphoblastic leukaemia using donor T cells in which three genes had been disabled by base editing, converting them into a CAR-T product that would not attack itself.³ The editing was done outside the body, which sidesteps the delivery problem entirely.

§Current state

As of 2026, base editing is in clinical trials rather than approved practice, and the trials divide into two families.

Ex vivo programmes edit cells outside the body and return them. A sickle cell disease candidate uses an adenine editor to recreate a naturally occurring promoter variant that keeps fetal haemoglobin switched on in adulthood — a different route to the same physiological end as Casgevy, which disrupts an erythroid enhancer of the repressor gene BCL11A instead. Both approaches share the burden that limits autologous stem cell therapy generally: patients need conditioning chemotherapy to clear marrow space, and that conditioning carries real mortality and infertility risk independent of the edit.

In vivo programmes deliver the editor systemically, almost always to the liver, using Lipid nanoparticles. The first in-human in vivo base editing trial, run by Verve Therapeutics, disabled PCSK9 in hepatocytes to lower LDL cholesterol permanently; a successor candidate from the same programme reported substantial cholesterol reductions in 2025. A separate liver programme has reported correction — as opposed to disruption — of a disease-causing point mutation in patients with alpha-1 antitrypsin deficiency, which if it holds up would be the first such correction in a person.

The bespoke-therapy case

In 2025 clinicians at the Children's Hospital of Philadelphia treated an infant with a severe urea-cycle disorder using a base editor designed for his specific mutation and delivered by lipid nanoparticle, from diagnosis to dosing in roughly six months.⁴ It is the clearest existing demonstration that editing can be personalised, and the clearest illustration that regulatory and manufacturing pathways for one-patient medicines do not yet exist.

§Limitations

The four accessible substitutions are the transitions — C to T, G to A, A to G, T to C. The eight transversions require different chemistry. Glycosylase-based editors that produce C-to-G changes exist but are less efficient and less clean, and for most transversions Prime editing remains the practical option.

Targeting is still constrained by the PAM requirement inherited from Cas9, which limits how the editing window can be positioned over a given base. Cargo size is a problem for viral delivery: a base editor plus guide RNA exceeds the packaging capacity of a single adeno-associated virus, forcing split-vector designs. And systemic delivery beyond liver and haematopoietic cells remains largely unsolved, which is the same constraint that limits Targeted drug delivery generally.

§Risks

Base editors avoid double-strand breaks but introduce a distinct failure mode: deaminase activity that does not depend on the guide RNA at all. Cytosine editors have been shown to cause genome-wide off-target single-nucleotide changes in mouse embryos and rice through Cas9-independent deamination of transiently exposed single-stranded DNA.⁵ Both editor classes can also edit cellular RNA, since the deaminases do not distinguish substrates as sharply as intended.⁶ Engineered variants reduce both effects but do not abolish them, and detecting scattered single-base changes is harder than detecting the indels left by nuclease off-target activity, because there is no cut site to look near.

Bystander edits raise a subtler issue for Somatic gene therapy: a silent bystander change in one cell type may not be silent in another, and the tissue is a mosaic of edit patterns rather than a uniform genotype.

For heritable use, the apparent tidiness of base editing has been used as an argument that Human germline editing is now safe enough to attempt. Most of the field rejects that inference. Embryo editing still produces mosaicism, the Cas9-independent off-target mechanism operates during the exact developmental window in question, and no whole-genome assay can certify the absence of scattered point mutations in a single embryo without destroying it. The regulatory posture that followed the He Jiankui affair has not shifted, and the frameworks surveyed in Governance of human genome editing treat editor chemistry as largely beside the point.

§Outlook

The near-term question is whether permanent single-dose edits to common risk genes — a lifetime cholesterol reduction from one infusion, for instance — can clear a safety bar set for chronic disease rather than for fatal rare disease. That bar is high, because the comparator is a cheap daily pill and the edit is irreversible. The technical frontier is delivery to tissues other than liver, editors small enough for single-vector packaging, and reliable assays for scattered off-target deamination. The economic frontier, shared with Gene therapy for aging and every other one-time genetic medicine, is whether a treatment designed for one child can ever be paid for, a problem that runs directly into Access and inequality and that no current reimbursement system is built to solve.

§See also

• Prime editing
• CRISPR–Cas9
• Epigenome editing
• Off-target effects in genome editing
• Lipid nanoparticles
• Casgevy
• Somatic gene therapy
• Human germline editing

§References

Footnotes

1. paperKomor, A. C., Kim, Y. B., Packer, M. S., Zuris, J. A., Liu, D. R. "Programmable editing of a target base in genomic DNA without double-stranded DNA cleavage." Nature, 2016. ↩

2. paperGaudelli, N. M. et al. "Programmable base editing of A•T to G•C in genomic DNA without DNA cleavage." Nature, 2017. ↩

3. paperChiesa, R. et al. "Base-edited CAR7 T cells for relapsed T-cell acute lymphoblastic leukemia." New England Journal of Medicine, 2023. ↩

4. paperMusunuru, K. et al. "Patient-specific in vivo gene editing to treat a rare genetic disease." New England Journal of Medicine, 2025.↩A report of one infant treated under an individualised protocol, not a trial; it establishes feasibility rather than efficacy.

5. paperZuo, E. et al. "Cytosine base editor generates substantial off-target single-nucleotide variants in mouse embryos." Science, 2019.↩The method compared edited and unedited cells descended from the same embryo, which is why it detected changes that sequencing predicted sites would miss.

6. paperGrünewald, J. et al. "Transcriptome-wide off-target RNA editing induced by CRISPR-guided DNA base editors." Nature, 2019. ↩