Gene therapy is used to treat diseases caused by faulty or missing genes, including blood disorders like sickle cell disease and hemophilia, inherited blindness, spinal muscular atrophy, and several types of blood cancer. As of 2026, the FDA has approved more than 30 gene and cell therapy products, with the majority targeting conditions that previously had no cure or only lifelong symptom management.
How Gene Therapy Works
At its core, gene therapy fixes a problem at the DNA level rather than managing symptoms with ongoing medication. There are three main approaches. Gene addition delivers a working copy of a gene into cells that have a broken or missing version, allowing those cells to produce the protein they couldn’t make before. Gene silencing switches off a gene that’s causing harm, essentially muting its instructions. Gene correction, often using CRISPR technology, edits the faulty DNA sequence directly, rewriting the genetic typo that causes disease.
Getting therapeutic genes into cells requires a delivery vehicle, and most approved therapies use modified viruses that have been stripped of their ability to cause illness. The two most common types work quite differently. AAV vectors (adeno-associated viruses) carry small DNA packages and deliver them directly into cells inside the body, through an injection or infusion. They don’t typically insert their cargo into the cell’s own genome, which makes them safe but means the effect can fade over time in cells that divide frequently. Lentiviral vectors carry larger genetic payloads and do integrate into the cell’s DNA, making them better suited for therapies where stem cells or immune cells are removed from the body, modified in a lab, and then returned to the patient.
Blood Disorders
Sickle cell disease was one of the first conditions to receive multiple gene therapy options. In December 2023, the FDA approved two gene therapies for sickle cell, including the first-ever CRISPR-based treatment. That therapy, Casgevy, works by editing patients’ blood stem cells to reactivate production of fetal hemoglobin, a form of hemoglobin that babies produce naturally but that gets switched off after birth. Fetal hemoglobin prevents red blood cells from sickling, and the results have been striking: of 31 patients with enough follow-up data, 93.5% were free from severe pain crises for at least 12 consecutive months after treatment.
Hemophilia B, a bleeding disorder caused by insufficient clotting factor, is treated with Hemgenix, a gene therapy that delivers a working gene for Factor IX. Before treatment, patients in clinical trials averaged about 4 bleeding episodes per year. After a single infusion, that dropped to roughly 2 per year, and patients’ Factor IX activity rose to an average of about 39% of normal levels, enough to significantly reduce spontaneous bleeding. For people accustomed to frequent infusions of clotting factor, a one-time treatment that produces lasting results represents a fundamental shift.
Inherited Vision Loss
Luxturna, approved in 2017, treats a specific form of inherited retinal dystrophy caused by mutations in both copies of the RPE65 gene. This condition progressively destroys vision, often starting with severe night blindness in childhood and advancing toward complete blindness. The therapy delivers a functional copy of RPE65 directly into retinal cells through a surgical injection beneath the retina. In clinical trials, treated patients showed statistically significant improvements in visual field at one year compared to untreated controls, measured by both full-field and central-threshold testing. The catch is specificity: only people with confirmed mutations in both copies of RPE65 are eligible, which represents a small fraction of all inherited blindness.
Spinal Muscular Atrophy
Spinal muscular atrophy (SMA) is a genetic disease in which motor neurons progressively die, leading to muscle weakness and, in its most severe form, death before age two. Zolgensma delivers a working copy of the SMN1 gene through a single intravenous infusion and is approved for children under two years old. Before gene therapy existed, babies with the most severe type of SMA rarely survived past infancy without ventilator support. Treated infants have achieved motor milestones like sitting and, in some cases, walking that would have been impossible without intervention. Early treatment, ideally before symptoms appear, produces the best outcomes, which is why SMA is now included in newborn screening panels across the United States.
Cancer
CAR-T cell therapy represents a different flavor of gene therapy: rather than fixing a defective gene, it engineers a patient’s own immune cells to recognize and attack cancer. T cells are collected from the patient’s blood, genetically modified in a lab to express a receptor that locks onto cancer cells, then multiplied and infused back. The first CAR-T therapy was approved in 2017 for children with acute lymphoblastic leukemia, and the list of approved uses has grown to include several blood cancers: non-Hodgkin lymphoma, multiple myeloma, follicular lymphoma, mantle cell lymphoma, and chronic lymphocytic leukemia.
The numbers in some cancers are remarkable. In one trial involving advanced follicular lymphoma, nearly 80% of patients had their cancer eliminated, and many remained cancer-free three years later. For large cell lymphoma, more than 30% of patients were alive with no detectable cancer five years after treatment. These are patients whose cancers had already resisted multiple rounds of conventional chemotherapy. CAR-T therapy is not yet widely used for solid tumors, though research in that area is active.
Other Conditions With Approved Therapies
The list of treatable conditions continues to expand. Elevidys targets Duchenne muscular dystrophy, a progressive muscle-wasting disease in boys. Hemophilia A has its own gene therapy (Roctavian) separate from the hemophilia B treatment. Lenmeldy treats metachromatic leukodystrophy, a rare brain disease in young children. Kebilidi addresses a different neurological condition affecting dopamine production. Bladder cancer that hasn’t responded to other treatments has a gene therapy option (Adstiladrin) that delivers a modified virus directly into the bladder. And Casgevy, the CRISPR-based sickle cell therapy, is also approved for transfusion-dependent beta thalassemia, another inherited blood disorder.
Safety and Side Effects
Gene therapy is not risk-free. A large meta-analysis of gene replacement therapies found that about 30% of patients experienced an immune-related adverse event. The most common was liver inflammation, occurring in roughly 24% of patients, though 97% of those cases were mild. Heart inflammation occurred in about 6% of patients, typically peaking around two weeks after treatment, and was also overwhelmingly non-serious. A more concerning complication, thrombotic microangiopathy (a clotting disorder affecting small blood vessels), appeared in about 5% of patients and carried significant health consequences when it did occur. Treatment-related deaths were reported at a rate of roughly 5%, primarily in patients who already had serious underlying conditions.
The timing of these side effects follows a predictable pattern. Clotting problems tend to appear within the first week, heart inflammation around week two, and liver complications can develop gradually over the first six months. This is why patients receiving gene therapy are monitored closely in the weeks and months following treatment.
For CAR-T therapies specifically, cytokine release syndrome is the most well-known risk. It happens when the engineered immune cells activate so aggressively that they trigger a systemic inflammatory response, causing fever, low blood pressure, and in severe cases, organ dysfunction. Treatment teams expect this and have protocols to manage it, but it means patients typically stay in or near the treatment center for several weeks after infusion.
Cost and Access
Gene therapies carry some of the highest price tags in medicine, with several treatments exceeding $1 million for a single dose. The logic from manufacturers is that a one-time cure replaces decades of ongoing treatment costs, but the upfront expense creates real barriers, particularly for Medicaid programs covering low-income patients who are disproportionately affected by conditions like sickle cell disease.
To address this, the Centers for Medicare and Medicaid Services launched the Cell and Gene Therapy Access Model, the first time the federal government has negotiated directly with manufacturers on behalf of state Medicaid agencies. The model uses outcomes-based agreements: manufacturers offer pricing discounts upfront and pay additional rebates if the therapy doesn’t deliver the promised health outcomes. The initial focus is on sickle cell disease treatments. Participating manufacturers are also required to cover fertility preservation services for patients, since the chemotherapy conditioning required before many gene therapies can affect fertility.