Gene Addition and Gene Therapy Explained
What gene therapy and gene addition are, how genes are delivered, which products the FDA has approved, and the serious risks and limits that come with them.
Content last updated . This page has not been medically reviewed.
Gene addition and other approaches
Gene therapies can work in several ways: replacing a disease-causing gene with a healthy copy, inactivating a disease-causing gene that is not working properly, or introducing a new or modified gene to help treat a disease. [2] "Gene addition" is the first of these in its simplest form: adding a working copy of a gene to stand in for the faulty one. [1]Gene therapy is different from gene editing, which changes DNA that is already in the cell. [1] See Gene Editing and CRISPR Explained and Gene Therapy vs Gene Editing.
How the gene gets into cells
The FDA lists several types of gene therapy products, including plasmid DNA, viral vectors and bacterial vectors. Viruses naturally deliver genetic material into cells, so some products use viruses that have been modified to remove their ability to cause infectious disease. [3] Details are in Viral Vectors in Gene Therapy and Lipid Nanoparticles and Non-Viral Delivery.What the evidence looks like
Evidence labels differ by product. For Zolgensma, an early trial gave a single intravenous AAV9 dose to 15 infants with spinal muscular atrophy type 1. All 15 were alive and event-free at 20 months, compared with 8% survival in a historical cohort, but there was no concurrent control group, and the authors said further studies were needed. [9] Comparing against historical records is weaker than a randomized trial. For more on research in rare conditions, see Rare-Disease Gene Therapy Research.What gene therapy is not
- It is not something to do at home. For "do-it-yourself" claims see DIY Gene-Therapy Claims.
- A clinic offering a treatment does not mean the treatment is approved. See Unapproved Clinics and Medical Tourism for Cell Therapies.
- A registered trial proves nothing about effectiveness by itself.
Frequently asked questions
Is gene therapy approved or still experimental?
Both. MedlinePlus says gene therapies are being used to treat a small number of diseases, while many more are being researched for safety and effectiveness. [1] The FDA keeps a list of the licensed products. [4]
Is gene therapy the same as CRISPR?
No. Gene therapy often adds or replaces a gene, while gene editing changes DNA already in the cell. [1]
Can gene therapy cause cancer?
Early studies showed serious risks including cancer, and the FDA has approved labeling changes for an increased risk of hematologic malignancy after Skysona. [11] [8] Risk differs by product.
Why are people followed for years afterward?
Gene therapies can produce permanent or long-acting changes, so the FDA recommends extended monitoring for delayed adverse events. [12]
Does a viral vector mean you get infected?
The viruses used have been modified to remove their ability to cause infectious disease. [3] They can still trigger immune reactions, as the risks above show. [10]
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 means a regulator has approved a named product for a named use; investigational means it is still being tested; preclinical means cells or animals only. More detail is in How We Label Evidence in Specialty Topics.
What is approved in the United States
| Product (approved medicine, FDA) | Indication as listed |
|---|---|
| Luxturna | Confirmed biallelic RPE65 mutation-associated retinal dystrophy [5] |
| Zolgensma | Children under two years of age with spinal muscular atrophy; an adeno-associated virus (AAV) vector-based product [6] |
| Elevidys | Duchenne muscular dystrophy, ambulatory patients 4 years and older with a confirmed DMD gene mutation; an AAV vector-based product [7] |
| Skysona | Boys 4 to 17 years old with early, active cerebral adrenoleukodystrophy and no matched stem-cell donor [8] |
Approval of one product says nothing about other products with similar names. Other approved examples and indications are on Approved Gene and RNA Therapies.
Risks and who should be careful
- Liver injury. Zolgensma has a boxed warning that acute serious liver injury can occur, and people with existing liver impairment may be at higher risk. [6]
- Elevidys. The FDA page lists a new boxed warning for acute serious liver injury and acute liver failure and a revised indication limited to ambulatory patients (14 November 2025), and notes an FDA investigation of deaths from acute liver failure in non-ambulatory patients (24 June 2025). [7]
- Blood cancer. The FDA approved labeling changes for an increased risk of hematologic malignancy after Skysona. [8]
- Immune reactions to high vector doses. A review reports fatalities in high-dose systemic AAV trials from liver, kidney, heart or lung failure, with immune responses to the vector playing a critical role. [10]
- Early history. The earliest gene therapy studies showed serious risks such as toxicity, inflammation and cancer; newer methods are less likely to cause these, but the techniques are new and some risks may not yet be known. [11]
Because effects may be long-lasting, the FDA recommends extended follow-up of people in gene therapy trials for delayed adverse events. [12] See Immune Responses to Gene Therapy and Off-Target Effects, Durability and Long-Term Monitoring.
Safety note
References
- MedlinePlus Genetics, What is gene therapy?. Read 2026-10-22.
- US FDA, What is Gene Therapy?. Read 2026-10-22.
- US FDA, What is Gene Therapy?. Read 2026-10-22.
- US FDA, Approved Cellular and Gene Therapy Products. Read 2026-10-22.
- US FDA, LUXTURNA product page. Read 2026-10-22.
- US FDA, FDA approves innovative gene therapy to treat pediatric patients with spinal muscular atrophy. Read 2026-10-22.
- US FDA, ELEVIDYS product page. Read 2026-10-22.
- US FDA, SKYSONA product page. Read 2026-10-22.
- Mendell JR et al. Single-Dose Gene-Replacement Therapy for Spinal Muscular Atrophy. N Engl J Med 2017. Read 2026-10-22.
- Lethal immunotoxicity in high-dose systemic AAV therapy. Mol Ther 2023. Read 2026-10-22.
- MedlinePlus Genetics, Is gene therapy safe?. Read 2026-10-22.
- US FDA, Long Term Follow-up After Administration of Human Gene Therapy Products. Read 2026-10-22.