Gene editing has entered clinical medicine, but the first approved CRISPR treatment does not edit genes directly inside the patient. Instead, cells are removed, modified in a laboratory, and returned to the body.
This approach is known as ex-vivo editing and provides scientists with significant control over the process. However, it also requires individualized manufacturing, specialized medical centers and, in some cases, intensive chemotherapy.
In-vivo editing takes a fundamentally different approach. Rather than removing cells, the genetic-editing machinery is delivered directly into the patient, where it must locate the appropriate tissue and modify cells inside the body.
Early clinical studies show that in-vivo editing can alter genes in the human liver after a single infusion. If the technology can be safely extended to other tissues, it could make gene editing simpler, more scalable and potentially accessible to more patients.
Gene therapy and gene editing are related but not identical
Gene therapy is a broad term describing treatments that add, replace, remove or regulate genetic material. Some gene therapies deliver a functional copy of a gene without changing the patient’s existing DNA.
Gene editing is a more targeted form of genetic medicine. Technologies such as CRISPR–Cas9, base editing and prime editing are designed to modify a specific genomic sequence or alter the activity of a selected gene.
A useful distinction is therefore:
- Gene addition introduces new genetic material.
- Gene editing changes or regulates DNA already present in the cell.
- Ex-vivo therapy modifies cells outside the body.
- In-vivo therapy delivers the treatment directly into the body.
Both gene addition and gene editing can be performed in vivo or ex vivo, depending on the disease and delivery system.
How Ex-vivo Editing Works
Ex-vivo editing begins by collecting a patient’s cells. These may be blood-forming stem cells, immune cells or another cell type that can survive outside the body.
The general process includes:
- Cells are collected from the patient.
- The desired cells are isolated and sometimes expanded.
- Gene-editing components are introduced in a laboratory.
- The cells are tested for identity, potency, purity and genetic changes.
- The patient may receive treatment to prepare the body for the modified cells.
- The edited cells are infused back into the patient.
The first approved CRISPR treatment, Casgevy, uses this model. Blood-forming stem cells are collected and edited with CRISPR–Cas9 to disrupt a regulatory region associated with BCL11A. This allows developing red blood cells to produce more fetal hemoglobin, which can compensate for the abnormal hemoglobin involved in sickle cell disease and beta-thalassemia.
The edited stem cells are returned after the patient receives myeloablative conditioning high-dose chemotherapy that makes room in the bone marrow for the modified cells.
Casgevy is now FDA-approved for eligible patients aged two years and older with sickle cell disease or transfusion-dependent beta-thalassemia.
Advantages of Editing Cells Outside the Body
The major advantage of ex-vivo editing is control. Researchers can examine the cells before they are returned to the patient.
This provides opportunities to:
- Measure how many cells were successfully edited
- Check cell viability and function
- Test for contamination
- Assess selected off-target changes
- Confirm that the final product meets manufacturing standards
- Avoid exposing unrelated organs to the editing machinery
Ex vivo treatment can also use delivery methods such as electroporation. During electroporation, a brief electrical pulse creates temporary openings in cell membranes so editing components can enter. This would be difficult to apply safely throughout the body.
The approach is particularly useful for cells that can be removed and returned without losing their function. Blood stem cells and immune cells are leading examples.
In cancer research, immune cells can be edited to recognize tumors, resist suppressive signals or persist longer after infusion. Researchers can test the modified cells before administration, providing a degree of quality control that is not possible when editing occurs entirely inside the body.
Limitations of Ex-vivo Treatment
Ex-vivo editing is complex and expensive. Every patient may require an individually manufactured cell product.
The process can involve:
- Cell collection procedures
- Transportation to a manufacturing facility
- Weeks of processing and quality testing
- Specialized transplant or cell-therapy centers
- Chemotherapy or other conditioning
- Hospitalization
- Risk of infection during immune recovery
- Long-term follow-up
Manufacturing failure or delays may occur if too few cells are collected, the cells are not edited efficiently, or the final product does not meet release standards.
Ex-vivo editing is also unsuitable for many tissues. Neurons, heart-muscle cells and cells embedded within the liver, lung or skeletal muscle generally cannot be removed, edited and returned in a clinically practical manner.
These limitations are driving interest in in vivo editing.
How In-vivo Gene Editing Works
In vivo editing packages the genetic instructions into a delivery vehicle. The therapy may then be administered through an intravenous infusion, injection into a particular organ, or another localized route.
The delivery system must complete several tasks:
- Protect the editing components from degradation.
- Travel to the intended tissue.
- Avoid excessive accumulation in unrelated organs.
- Enter the correct cells.
- Release its cargo.
- Allow the editing machinery to reach the nucleus.
- Produce enough editing to create a clinical benefit.
This is a much more difficult delivery problem than placing isolated cells in a laboratory dish.
Once the treatment has been administered, clinicians generally cannot remove the edited cells or inspect them individually. Safety must therefore be established through extensive preclinical testing, careful dose escalation, blood-based measurements, tissue sampling when possible, and long-term clinical monitoring.
The Liver Has Become the First Major Target
The most advanced in vivo editing programs target the liver. This is partly because the liver naturally removes many particles from the bloodstream, making it relatively accessible to lipid nanoparticles.
Lipid nanoparticles can temporarily carry messenger RNA encoding an editing enzyme together with a guide RNA. After entering liver cells, the messenger RNA is translated, the editor performs its task, and the components are gradually degraded. The DNA change, however, may remain.
In 2021, researchers reported the first systemic in vivo CRISPR treatment in people with hereditary transthyretin amyloidosis. A single infusion of NTLA-2001 delivered CRISPR–Cas9 components to the liver and disrupted the TTR gene. Early participants experienced dose-dependent reductions in the abnormal transthyretin protein responsible for the disease.
Another investigational treatment, NTLA-2002, edits KLKB1 in liver cells to reduce plasma kallikrein, a protein involved in hereditary angioedema. Early-stage results showed substantial reductions in kallikrein and encouraging decreases in attacks, although larger studies are needed.
These therapies do not correct a mutation letter by letter. Instead, they disable a gene whose reduced activity is expected to provide a therapeutic benefit.
Editing Genes Associated with Cholesterol
In vivo editing is also being studied for cardiovascular risk. People who naturally carry inactive versions of genes such as PCSK9 or ANGPTL3 tend to have lower cholesterol levels. Researchers are attempting to reproduce this protective effect by permanently disrupting those genes in liver cells. A phase 1 study of CRISPR–Cas9 targeting ANGPTL3 reported dose-dependent reductions in circulating ANGPTL3 after a single treatment. Base editing is also being tested against PCSK9. Unlike conventional CRISPR–Cas9, a base editor can change a selected DNA letter without creating a standard double-strand break.
In a 2026 phase 1 study, VERVE-102 was delivered intravenously to 35 adults with heterozygous familial hypercholesterolemia or premature coronary artery disease. Treatment produced dose-dependent reductions in PCSK9 and LDL cholesterol. At the highest dose, average LDL cholesterol fell by approximately 62%, and reductions appeared durable throughout available follow-up.
These results are promising, but the studies remain early. Reducing LDL cholesterol is not the same as proving that a therapy prevents heart attacks or strokes. Larger and longer trials are required, particularly because existing cholesterol-lowering treatments are effective and reversible.
Comparing ex-vivo and in-vivo editing
|
Consideration |
Ex-vivo editing |
In-vivo editing |
|
Where editing occurs |
In a laboratory |
Inside the patient |
|
Cells commonly targeted |
Blood stem cells and immune cells |
Currently led by liver cells |
|
Product |
Patient-specific edited cells |
Standardized delivery formulation |
|
Ability to test cells before treatment |
Relatively high |
Limited |
|
Systemic exposure to editor |
Usually lower |
Depends on delivery and distribution |
|
Manufacturing |
Complex and individualized |
Potentially more scalable |
|
Conditioning treatment |
Often required for stem-cell therapies |
May not be required |
|
Ability to reach solid organs |
Limited |
Potentially broad |
|
Ability to remove an unwanted edit |
Usually impossible after infusion |
Usually impossible after editing |
|
Key challenge |
Manufacturing and transplantation |
Safe, tissue-specific delivery |
Neither method is universally superior. The best approach depends on whether the cells can be removed, whether they will survive manufacturing, and whether an in vivo delivery system can reach the correct tissue safely.
Delivery is the central challenge
The editing enzyme may be programmable, but the delivery vehicle determines where it goes.
Lipid nanoparticles currently perform well in the liver but do not naturally provide efficient access to every tissue. Researchers are modifying lipid chemistry and adding targeting molecules to direct nanoparticles toward the lungs, muscles, immune cells, bone marrow and other sites.
Viral vectors, particularly adeno-associated viruses, can reach certain organs efficiently. However, they have limitations:
- Restricted cargo capacity
- Immune responses against the vector
- Difficulty giving repeat doses
- Possible liver toxicity
- Prolonged production of editing components
- Variable distribution among tissues
Long-lasting expression can be useful for gene addition, but it may be less desirable for editing. Once the intended edit has been made, continued editor activity may increase the opportunity for unintended changes.
Virus-like particles are another emerging option. These particles borrow viral delivery mechanisms but do not contain a viral genome. In 2025, researchers reported that engineered virus-like particles could edit human blood-forming stem and progenitor cells in preclinical models while reducing exposure to human liver cells.
The results suggest that blood stem cells may eventually be edited directly inside the bone marrow. However, this research remains preclinical and has not established safety or effectiveness in patients.
Can in vivo editing replace bone marrow procedures?
One ambitious goal is to treat sickle cell disease and beta-thalassemia by delivering editors directly to blood-forming stem cells inside the body.
If successful, this could reduce or eliminate several burdens associated with ex-vivo therapy:
- Stem-cell collection
- Individualized manufacturing
- Transportation of cells between facilities
- Lengthy production timelines
- Some forms of conditioning chemotherapy
- Extended hospitalization
The challenge is reaching true long-term blood stem cells in the bone marrow while avoiding mature blood cells, the liver and other tissues. Researchers must also edit enough stem cells for the corrected cells to produce a lasting benefit.
Even if direct delivery succeeds, some form of conditioning may still be needed to give edited cells a competitive advantage. Less toxic antibody-based conditioning approaches are being studied as alternatives to conventional chemotherapy.
Off-target and on-target safety
In both ex-vivo and in-vivo editing, the editor may modify a DNA sequence resembling the intended target. These off-target changes could be harmless, but they could also disrupt genes involved in normal cellular control.
Editing the correct target can also produce unexpected outcomes, including:
- Large deletions
- DNA insertions
- Chromosomal rearrangements
- Unexpected repair patterns
- Different edits among individual cells
Ex-vivo editing allows samples of the final cell product to be examined before treatment. This does not guarantee that every rare abnormality will be detected, but it provides an additional quality-control step.
With in vivo therapy, clinicians usually estimate editing indirectly through changes in blood proteins or disease markers. A liver biopsy may provide more direct information, but only from a small area of one organ.
In vivo delivery also creates biodistribution concerns. If the delivery vehicle enters an unintended tissue, editing could occur in cells that were never meant to be targeted. Developers must therefore study both genetic specificity and tissue specificity.
Immune responses and repeat dosing
The immune system may recognize components of an in vivo therapy as foreign. Possible targets include:
- The delivery vehicle
- The CRISPR-associated protein
- Newly produced or altered proteins
- Cells displaying evidence of editing
Some people may already have antibodies or immune cells that recognize Cas proteins because these enzymes originate from bacteria commonly encountered by humans.
Temporary delivery through messenger RNA may reduce the period of exposure, but it does not eliminate the possibility of immune reactions. Lipid nanoparticles can also cause infusion-related responses or temporary elevations in liver enzymes.
Repeat dosing may be difficult, particularly with viral vectors, because the first dose can produce neutralizing antibodies. Gene-editing treatments are often designed as one-time interventions, placing additional pressure on the initial dose to be both safe and effective.
Durability: benefit and responsibility
A permanent edit could provide years or decades of benefit after one treatment. This is particularly valuable for genetic diseases that otherwise require lifelong therapy.
Durability depends on the cells being edited. An edit in a long-lived liver cell may persist for years but could be diluted if the cells are replaced. Editing a self-renewing stem cell may allow the change to be passed to many generations of daughter cells.
The same permanence creates uncertainty. Conventional medication can often be stopped or adjusted. A DNA edit may not be reversible if an unexpected effect appears years later.
Long-term monitoring is therefore essential. Researchers must evaluate not only whether the effect continues, but also whether edited cells behave normally and whether delayed complications emerge.
Could in vivo treatment improve accessibility?
In vivo therapy could eventually be easier to distribute than individualized ex vivo cell products. A standardized formulation could theoretically be manufactured in batches, stored and administered at more treatment centers.
This may reduce:
- Manufacturing time
- Patient-specific production failures
- Cell-transport logistics
- Hospital stays
- Treatment costs
- Dependence on transplant infrastructure
However, easier administration does not guarantee equitable access. In-vivo gene-editing therapies may still be extremely expensive and require genetic diagnosis, specialist evaluation and lifelong monitoring.
The greatest burden of some target diseases occurs in regions with limited access to genetic testing and advanced medical care. Accessibility must therefore be incorporated into product design, manufacturing and pricing strategies from the beginning.
The Bottom Line
Ex-vivo editing provides greater control because cells can be modified and tested before being returned to the patient. It is already producing meaningful clinical benefits in blood disorders, but it requires individualized manufacturing and intensive medical care.
In-vivo editing aims to deliver the treatment directly to cells inside the body. Early trials show that liver genes can be edited after a single intravenous infusion, opening the possibility of durable treatments for inherited disease, cardiovascular risk and other conditions.
The field’s next major challenge is not simply designing better editors. It is learning how to deliver them to the right cells, in the right amount, without affecting unintended tissues.
If researchers can solve that problem, gene editing may move from a highly specialized cell-manufacturing procedure toward a more scalable form of medicine. Until then, in-vivo editing remains one of biotechnology’s most promising—and carefully watched—clinical frontiers.
This article is intended for scientific education and does not replace individualized medical advice. Most in vivo gene-editing therapies discussed remain investigational and are not approved for routine clinical use.
References
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