Gene Editing vs Gene Therapy: What's the Difference?

Two Approaches, One Goal

If you have been reading about genetic medicine, you have probably seen the terms "gene therapy" and "gene editing" used almost interchangeably. News headlines blur the line constantly. But these two approaches are fundamentally different — in how they work, what they can treat, and what they mean for patients.

Understanding the difference matters. As more genetic treatments reach patients, knowing whether a therapy adds a gene or fixes one can help you make sense of the science, the news, and — if you or someone you love has a genetic condition — the treatment options ahead.

This article breaks down both approaches in plain language, compares them side by side, and explains when each one makes more sense.

What Is Gene Therapy?

Gene therapy is the older of the two approaches. The core idea is simple: if a patient's body cannot make a critical protein because the gene responsible is broken, give them a working copy of that gene.

The Spare Tire Analogy

Think of your genome as a car. A genetic disease is like driving on a flat tire. Gene therapy does not patch the flat tire. Instead, it puts a spare tire in the trunk and hooks it up so the car can keep moving. The original flat tire is still there — it has not been repaired — but the new spare does the job.

In biological terms, a working copy of the gene is packaged inside a delivery vehicle (usually a harmless virus) and introduced into the patient's cells. Once inside, the new gene copy starts producing the missing protein. The original mutated gene remains in the DNA, untouched.

How Gene Therapy Works, Step by Step

Identify the faulty gene: Scientists determine which gene is broken and what protein it should be making.
Build a working copy: A functional version of the gene is synthesized in the lab.
Package it in a vector: The gene is loaded into a delivery vehicle — most commonly an adeno-associated virus (AAV). AAVs are small, non-disease-causing viruses that are very good at entering human cells.
Deliver it to the patient: The vector is injected into the patient's body (in vivo) or into their cells in a lab dish (ex vivo), depending on the therapy.
The new gene goes to work: Once inside the target cells, the new gene begins producing the missing or defective protein.

The original mutation stays in the patient's DNA. The new gene copy sits alongside it, often in a separate location, doing the work the broken gene cannot.

A Real-World Example: Zolgensma

Zolgensma (onasemnogene abeparvovec) is one of the most well-known gene therapies. It treats spinal muscular atrophy (SMA), a devastating genetic disease where babies lose the ability to move, swallow, and eventually breathe because they lack a working copy of the SMN1 gene.

Zolgensma delivers a functional SMN1 gene using an AAV9 vector, given as a single intravenous infusion. The working gene copy enters motor neurons and begins producing the SMN protein that these cells need to survive.

Before Zolgensma, most children with the severest form of SMA did not survive past age two. With treatment, many are now sitting, crawling, and in some cases walking. It was a landmark moment when the FDA approved Zolgensma in 2019.

But the mutated SMN1 gene is still there. Zolgensma does not fix it. It works around it.

What Is Gene Editing?

Gene editing takes a different approach entirely. Instead of adding a new gene copy, it goes into the patient's existing DNA and makes a precise change — fixing, disabling, or altering a specific gene.

The Tire Patch Analogy

Back to the car analogy. Gene editing does not add a spare tire. It patches the flat tire itself. It finds the exact spot where the damage is, repairs it, and gets the original tire working again.

In biological terms, a molecular tool (most famously CRISPR-Cas9) is sent into the patient's cells. This tool locates a specific sequence of DNA, cuts it, and either removes, replaces, or corrects the problematic sequence. The result is a permanent change to the patient's own genome.

How Gene Editing Works, Step by Step

Identify the target: Scientists pinpoint the exact DNA sequence that needs to be changed.
Design the editing tool: A guide molecule (guide RNA, in CRISPR) is designed to match the target sequence precisely.
Deliver the editor: The editing machinery — the guide RNA and the cutting protein (Cas9, in CRISPR) — is delivered into the patient's cells, either directly in the body or in cells removed from the patient.
The edit happens: The tool finds the target DNA, cuts it, and the cell's own repair machinery (or a provided template) makes the desired change.
The genome is permanently altered: The patient's own DNA now reads differently at that location. No new gene has been added — the existing one has been fixed or modified.

A Real-World Example: Casgevy

Casgevy (exagamglogene autotemcel), developed by Vertex Pharmaceuticals and CRISPR Therapeutics, became the first CRISPR-based therapy to receive FDA approval in December 2023. It treats sickle cell disease (SCD) and transfusion-dependent beta-thalassemia (TDT).

Casgevy works by editing the BCL11A gene in a patient's own bone marrow stem cells. BCL11A normally acts as a switch that turns off fetal hemoglobin production after birth. By disabling this switch with CRISPR, the cells resume producing fetal hemoglobin — a healthy form of the protein that compensates for the defective adult hemoglobin causing the disease.

The patient's stem cells are removed, edited in the lab, and infused back after chemotherapy conditioning. In clinical trials, over 93% of sickle cell patients who received Casgevy were free of painful vaso-occlusive crises for at least 12 months.

Casgevy does not add a new gene. It changes the patient's existing DNA.

Side-by-Side Comparison

Here is how gene therapy and gene editing stack up across the factors that matter most:

Feature	Gene Therapy	Gene Editing
Mechanism	Adds a new working gene copy	Modifies the patient's existing DNA
Original mutation	Left untouched	Corrected, disabled, or altered
Precision	Moderate — the new gene integrates somewhat randomly (AAV) or stays episomal	High — targets a specific DNA sequence
Permanence	Variable — episomal AAV can be lost over time as cells divide	Permanent — the change is in the genome and passed to daughter cells
Re-dosing	May be needed if effects wane; immune response to viral vector can complicate repeat dosing	Typically a one-time treatment
Delivery vehicles	AAV, lentivirus, lipid nanoparticles	CRISPR-Cas9/Cas12/Cas13, base editors, prime editors (delivered via virus, LNP, or electroporation)
Gene size limit	AAV has a ~4.7 kb packaging limit; lentivirus ~8 kb	No strict gene size limit (editing happens in place)
Off-target risk	Low (new gene goes to a specific tissue, not a specific DNA spot)	Present — the editor could cut unintended genomic sites
Cost (approximate)	$500K to $3.5M per treatment	$2.2M for Casgevy (the only approved gene editing therapy so far)
FDA-approved products	30+ (as of early 2026)	1 — Casgevy (approved December 2023)
Maturity	More established; first approval in 2017 (Luxturna)	Newer; first approval in 2023 (Casgevy)

When Gene Therapy Makes More Sense

Gene therapy is often the better choice in certain situations:

Large Gene Replacement

Some diseases involve genes that are simply too large or too complex to "edit" in a practical way. Duchenne muscular dystrophy (DMD), for example, is caused by mutations in the dystrophin gene — one of the largest genes in the human genome. Current gene therapy strategies deliver a shortened but functional "micro-dystrophin" gene. Editing the full gene precisely would be extraordinarily difficult with today's tools.

Loss-of-Function Mutations

When a disease is caused by a gene that simply does not work — it produces no protein or a non-functional one — adding a working copy is conceptually straightforward. This is the classic gene therapy scenario. You are not trying to fix a subtle error. You are giving the cell something it completely lacks.

Examples include:

Zolgensma for SMA (missing SMN1 protein)
Luxturna for inherited retinal dystrophy (missing RPE65 protein)
Hemgenix for hemophilia B (missing Factor IX protein)
Elevidys for DMD (truncated dystrophin)

When the Target Tissue Is Hard to Edit

Some organs are easier to reach with viral vectors than with editing tools. The liver, for instance, is an excellent target for AAV-based gene therapy because AAVs naturally home to liver cells after intravenous injection. Editing tools face more delivery challenges in some tissue types.

When Gene Editing Makes More Sense

Gene editing has distinct advantages in other scenarios:

Gain-of-Function Mutations

Some genetic diseases are not caused by a missing protein but by a protein that is toxic or overactive. In these cases, adding another gene copy would not help — you need to silence or correct the problematic one. Gene editing can do this by disabling the mutant gene or fixing the specific mutation.

Huntington's disease is a prominent example. The disease is caused by an expanded CAG repeat in the huntingtin gene that produces a toxic protein. Adding a normal copy would not stop the toxic one from being made. Editing or silencing the mutant allele is the more logical strategy.

Dominant Negative Mutations

When a mutated gene produces a protein that actively interferes with the normal protein (a "dominant negative" effect), gene therapy's add-a-copy approach can fall short. The defective protein is still being produced and still causing harm. Gene editing can correct or knock out the mutant copy, removing the source of the problem.

Precision Corrections

Some diseases are caused by a single known point mutation — one "letter" of DNA is wrong. Base editing and prime editing (advanced forms of gene editing) can correct these single-letter mistakes with surgical precision, without cutting both DNA strands. This is like fixing a single typo in a book rather than pasting in an entirely new chapter.

Avoiding Immune Responses to Viral Vectors

A significant challenge with gene therapy is the immune response to viral vectors. Many patients have pre-existing antibodies to AAVs from prior natural exposure, which can neutralize the therapy before it reaches its target. Gene editing tools can be delivered using non-viral methods like lipid nanoparticles (LNPs) or electroporation, potentially avoiding this problem.

The Regulatory Landscape

Gene therapy has a significant head start in the clinic. As of early 2026, there are more than 30 FDA-approved gene therapy products, including:

Luxturna (2017) — inherited retinal dystrophy
Zolgensma (2019) — spinal muscular atrophy
Abecma (2021) — multiple myeloma (CAR-T)
Carvykti (2022) — multiple myeloma (CAR-T)
Hemgenix (2022) — hemophilia B
Elevidys (2023) — Duchenne muscular dystrophy
Roctavian (2023) — hemophilia A (EU-approved; later withdrawn from EU market)
Multiple CAR-T cell therapies for blood cancers (Kymriah, Yescarta, Tecartus, Breyanzi)

Gene editing, by contrast, has exactly one approved therapy: Casgevy, approved in December 2023 for sickle cell disease and beta-thalassemia.

This does not mean gene editing is less promising — it simply had a later start. CRISPR was only discovered as a programmable editing tool in 2012, while gene therapy clinical trials have been running since the early 1990s. The gene editing pipeline is growing rapidly, with dozens of clinical trials underway for conditions including hereditary angioedema, high cholesterol (PCSK9 editing), HIV, certain cancers, and more.

The Costs

Both approaches are expensive — and controversially so.

Gene therapy prices have set records in the pharmaceutical world. Zolgensma costs approximately $2.25 million for a single dose. Hemgenix was priced at $3.5 million, making it briefly the most expensive drug in the world. The rationale is that these are one-time (or very infrequent) treatments for diseases that would otherwise require a lifetime of costly care.

Casgevy, the lone approved gene editing therapy, is priced at approximately $2.2 million. The cost reflects not just the editing technology but the complex procedure involved — stem cell collection, laboratory editing, chemotherapy conditioning, and hospital stays.

Both gene therapy and gene editing companies argue that their treatments are cost-effective when compared to the lifetime cost of managing chronic genetic diseases. A patient with severe sickle cell disease, for example, may accumulate $1.6 million or more in healthcare costs over a lifetime. Whether payers and patients agree with this value proposition remains one of the biggest challenges for the field.

Safety Considerations

Both approaches carry risks, but the risk profiles are different.

Gene Therapy Risks

Immune reactions: The viral vectors used in gene therapy can trigger immune responses, sometimes severe. In rare cases, this has been fatal — as in the tragic death of Jesse Gelsinger in a 1999 clinical trial.
Insertional mutagenesis: Some vectors (particularly lentiviruses and early retroviral vectors) integrate into the genome semi-randomly. If the new gene lands in or near a cancer-related gene, it can potentially cause cancer. This happened in early gene therapy trials for X-linked severe combined immunodeficiency (X-SCID), where several patients developed leukemia.
Durability uncertainty: AAV-delivered genes typically remain as episomes (separate from the chromosome). In dividing cells, these can be lost over time, potentially requiring re-dosing.

Gene Editing Risks

Off-target edits: The editing tool might cut or modify DNA at locations other than the intended target. While CRISPR technology has become far more precise, off-target effects remain a safety concern that requires careful screening.
On-target but unintended effects: Even at the correct location, the DNA repair process after a cut can introduce unexpected insertions or deletions.
Mosaicism: Not every cell in the treated population may be edited, leading to a mix of edited and unedited cells.
Conditioning toxicity: For therapies like Casgevy that require myeloablative chemotherapy before infusion of edited cells, the conditioning regimen itself carries significant risks including infection and infertility.

The Future: Convergence

One of the most exciting trends in genetic medicine is the convergence of gene therapy and gene editing into hybrid approaches. The line between them is already blurring.

Gene Editing Delivered by Gene Therapy Vectors

Several clinical programs use AAV vectors — a classic gene therapy tool — to deliver CRISPR components. This combines gene therapy's proven delivery with gene editing's precision. For example, in vivo gene editing approaches for liver diseases often package CRISPR guide RNAs and Cas9 inside AAV vectors for delivery.

Base Editing and Prime Editing

These next-generation editing tools can make precise changes without cutting both strands of DNA, reducing the risk of unwanted mutations. Base editors can swap one DNA letter for another. Prime editors can insert, delete, or replace short DNA sequences. Both promise even more precise corrections than traditional CRISPR-Cas9.

Companies like Beam Therapeutics (base editing) and Prime Medicine (prime editing) are advancing clinical programs that represent this next wave.

Epigenetic Editing

An emerging approach does not change the DNA sequence at all but instead modifies how genes are expressed — turning them up or down without altering the underlying code. This could offer the precision of gene editing with even fewer permanent risks, since the DNA sequence itself remains intact.

In Vivo Gene Editing

Most current gene editing therapies work ex vivo — cells are removed, edited in the lab, and returned to the patient. The future likely belongs to in vivo gene editing, where the editing tools are delivered directly into the body. Intellia Therapeutics has shown promising results with in vivo CRISPR editing for transthyretin amyloidosis (ATTR) and hereditary angioedema, using lipid nanoparticles to deliver the editing machinery to the liver.

A Quick Summary for Beginners

If you take away one thing from this article, let it be this:

Gene therapy = adding a new copy of a gene (spare tire)
Gene editing = fixing the existing gene (patching the tire)

Both aim to treat genetic diseases at their root cause. Gene therapy has a longer track record and more approved products. Gene editing is newer, potentially more precise, and growing fast.

Neither is universally "better." The right approach depends on the disease, the gene involved, the mutation type, and the available delivery methods. Many patients will benefit from therapies that combine elements of both.

The era of genetic medicine is just beginning. Understanding these two foundational approaches puts you in a much better position to follow the science — and the breakthroughs — as they come.

Sources & Further Reading

U.S. Food and Drug Administration. "Approved Cellular and Gene Therapy Products." FDA.gov
Doudna, J.A. & Charpentier, E. "The new frontier of genome engineering with CRISPR-Cas9." Science, 346(6213), 2014.
Mendell, J.R. et al. "Single-Dose Gene-Replacement Therapy for Spinal Muscular Atrophy." New England Journal of Medicine, 377:1713-1722, 2017.
Frangoul, H. et al. "CRISPR-Cas9 Gene Editing for Sickle Cell Disease and Beta-Thalassemia." New England Journal of Medicine, 384:252-260, 2021.
Gillmore, J.D. et al. "CRISPR-Cas9 In Vivo Gene Editing for Transthyretin Amyloidosis." New England Journal of Medicine, 385:493-502, 2021.
National Institutes of Health. "What is gene therapy?" MedlinePlus Genetics
High, K.A. & Roncarolo, M.G. "Gene Therapy." New England Journal of Medicine, 381:455-464, 2019.
Vertex Pharmaceuticals. "Casgevy (exagamglogene autotemcel) Prescribing Information." 2023.
American Society of Gene & Cell Therapy (ASGCT). "Gene and Cell Therapy FAQ." ASGCT.org