Gene therapy helps humans by treating diseases at their genetic root, either replacing a faulty gene, silencing a harmful one, or adding a new gene that the body needs to function properly. Rather than managing symptoms for a lifetime, gene therapy aims to fix the underlying problem in a single treatment. The FDA has now approved dozens of gene and cell therapy products, targeting conditions from inherited blindness and blood disorders to aggressive cancers that don’t respond to standard treatment.
How Gene Therapy Gets Into Your Cells
The core challenge of gene therapy is delivering a working copy of genetic instructions into the right cells. The most common approach uses modified viruses as delivery vehicles. Scientists strip these viruses of the components that cause illness and load them with therapeutic genes instead. The virus does what viruses naturally do: it enters your cells and deposits its genetic cargo. But instead of causing infection, it delivers a treatment.
The two most widely used viral carriers work differently. Adeno-associated viruses (AAVs) deliver their genetic material into cells without permanently inserting it into your DNA. Instead, the therapeutic gene forms small circular structures inside the cell nucleus that continue producing the needed protein for months or even years, particularly in cells that don’t divide frequently, like nerve cells and heart muscle cells. Lentiviral vectors take a different approach: they actually integrate the new gene into your existing DNA, creating a permanent change that gets passed along every time the cell divides. This makes lentiviruses especially useful for treating blood disorders, where you need the corrected gene to persist through generations of new blood cells. The tradeoff is a small risk that the gene lands in the wrong spot in the genome, potentially disrupting other important genes.
Treating Blood Disorders With a Single Infusion
Some of gene therapy’s most dramatic results have come in blood disorders that previously required lifelong treatment. Hemophilia B, a condition where the blood doesn’t clot properly due to a missing clotting protein, traditionally requires regular intravenous infusions of the missing factor, sometimes multiple times per week for an entire lifetime.
A gene therapy called Hemgenix changed that equation. In its pivotal trial, 96% of participants were able to stop their routine clotting factor infusions entirely after a single treatment. Their need for infusions dropped by 97%, and at 18 months post-treatment, their bodies were producing the clotting protein at an average level of 37% of normal, up from essentially zero. That level is enough to dramatically reduce bleeding episodes and, for many patients, eliminate the burden of constant treatment.
Sickle cell disease represents another major breakthrough. The FDA approved two gene therapies for sickle cell in December 2023, including Casgevy, the first treatment based on CRISPR gene-editing technology. Casgevy works by editing a patient’s own blood stem cells so they produce a form of hemoglobin that prevents red blood cells from sickling. Among 31 evaluable patients, 93.5% were free of severe pain crises for at least 12 consecutive months after treatment. For people who previously experienced agonizing episodes that sent them to the emergency room multiple times a year, that kind of result is life-changing.
Restoring Vision in Inherited Blindness
Luxturna, approved in 2017, treats a rare form of inherited retinal dystrophy caused by mutations in the RPE65 gene. Without a functioning copy of this gene, the light-sensing cells in the retina gradually stop working, leading to severe vision loss that typically begins in childhood.
The treatment involves injecting a working copy of the RPE65 gene directly beneath the retina. In clinical studies, patients experienced measurable improvements in light sensitivity, and eight out of nine patients in one study group reported being satisfied with their visual outcomes. Younger patients showed particularly notable improvements in their ability to navigate in dim lighting, a task that was previously impossible for them. The results have held up over years of follow-up, though patients with more advanced disease at the time of treatment tend to benefit less.
Fighting Cancer With Reprogrammed Immune Cells
CAR-T cell therapy is a form of gene therapy that reprograms a patient’s own immune cells to recognize and attack cancer. Doctors collect immune cells from the patient’s blood, genetically modify them in a lab to target a specific protein found on cancer cells, then infuse the supercharged cells back into the patient. Multiple CAR-T products are now approved for blood cancers including certain types of lymphoma, leukemia, and multiple myeloma.
For diffuse large B-cell lymphoma, the most common type of aggressive lymphoma, CAR-T therapy has achieved complete remission in 59% of patients, with a median response lasting 8.3 months. These are patients whose cancers had already failed to respond to other treatments, making any sustained remission significant. Several CAR-T therapies are now available, each targeting slightly different cancers or stages of disease.
What the Recovery and Risks Look Like
Gene therapy isn’t a simple injection and done. Many treatments require preparation, monitoring, and management of side effects. For blood disorder therapies like Casgevy, patients first undergo chemotherapy to clear out their existing bone marrow and make room for the corrected cells. This means a hospital stay and a recovery period similar to a bone marrow transplant.
For treatments delivered by viral vectors, the most common concern is the immune system reacting to the delivery virus. In the clinical program for a Duchenne muscular dystrophy gene therapy, liver inflammation occurred in 36% of patients, typically appearing four to eight weeks after infusion. Most cases resolved on their own or with steroid treatment. Rarer complications included heart inflammation (1% of patients) and muscle inflammation (1% of patients). To reduce these risks, patients typically take steroids for at least 60 days after treatment, with the dose gradually tapered down.
The severity of side effects varies by condition and treatment. No long-term safety concerns have emerged over several years of follow-up in muscular dystrophy trials, but gene therapy remains relatively new, and regulators require ongoing monitoring. For conditions where the therapy integrates new DNA into a patient’s genome, there is a theoretical concern about disrupting other genes, though this has not been a significant clinical problem with modern vector designs.
The Cost Question
Gene therapies carry staggering price tags, often around $2 million for a single treatment. That number produces sticker shock, but the comparison point matters. Sickle cell disease, for example, generates enormous costs over a lifetime: repeated hospitalizations, chronic pain management, organ damage, lost productivity, and shortened life expectancy. A cost-effectiveness analysis found that at a $2 million price, gene therapy for sickle cell disease costs roughly $126,000 per quality-adjusted life year gained when accounting for broader societal benefits like reduced lost work and caregiver burden. Depending on the analysis model and which outcomes are measured, value-based pricing estimates range from $1 million to $2.5 million, suggesting the current price point falls within a defensible range, if barely.
The practical barrier is that insurance coverage and payment structures weren’t designed for one-time, multimillion-dollar cures. Many patients face delays, denials, or complex negotiations before accessing approved therapies. Some manufacturers have introduced outcomes-based payment models, where insurers pay the full cost only if the therapy works as expected.
Conditions Gene Therapy Can Now Treat
The list of approved gene therapies has grown rapidly. Beyond the conditions already discussed, approved products now cover:
- Inherited retinal dystrophy (Luxturna)
- Spinal muscular atrophy (Itvisma/Zolgensma), which delivers a working copy of the survival motor neuron gene to infants and young children
- Cerebral adrenoleukodystrophy (Skysona), a devastating brain condition in boys
- Metachromatic leukodystrophy (Lenmeldy), another progressive brain disorder
- Hemophilia A (Roctavian) and Hemophilia B (Hemgenix and Beqvez)
- Beta-thalassemia and sickle cell disease (Casgevy, Lyfgenia)
- Duchenne muscular dystrophy (Elevidys)
- Bladder cancer (Adstiladrin)
- Multiple types of blood cancer (Kymriah, Yescarta, Tecartus, Breyanzi, Abecma, Carvykti)
Many of these conditions had no effective treatment before gene therapy existed. For families dealing with a diagnosis like spinal muscular atrophy or metachromatic leukodystrophy, these therapies represent the difference between progressive decline and a chance at a functional life. The pace of approvals has accelerated, with researchers now working on gene therapies for conditions affecting far larger populations, including metabolic disorders and certain cardiovascular diseases.