Gene therapy treats disease by changing the genetic instructions inside your cells. Instead of managing symptoms with daily medication, it targets the root cause: a faulty or missing gene. Some approaches add a working copy of a gene, others silence a harmful one, and newer techniques can edit DNA directly. The result, in many cases, is a one-time treatment that can produce lasting effects.
Three Ways to Fix a Gene
Not all gene therapies do the same thing to your DNA. There are three broad strategies, and understanding them helps make sense of the different treatments now available.
Gene addition is the most straightforward. If a disease is caused by a gene that doesn’t work, a therapy can deliver an extra, functional copy. Your cells read the new copy and start producing the protein they were missing. For sickle cell disease, for example, an added hemoglobin gene lets cells produce normal, non-sickling hemoglobin.
Gene silencing doesn’t add anything. Instead, it switches off a gene that’s causing problems. In another sickle cell approach, the therapy silences a gene that blocks production of fetal hemoglobin, a type of hemoglobin that works perfectly well but gets shut down after birth. Once that blocking gene is silenced, fetal hemoglobin production restarts.
Gene correction goes a step further by repairing the exact mutation responsible for a disease, restoring the original gene to its proper sequence. This is the most precise strategy and the one most closely tied to newer gene-editing tools like CRISPR.
How Genes Get Into Your Cells
The challenge isn’t just deciding what to do to a gene. It’s getting the instructions inside the right cells. DNA can’t simply be injected into your bloodstream and absorbed. It needs a delivery vehicle, and the most common ones are modified viruses.
Viruses evolved to inject genetic material into cells. Scientists have spent decades stripping dangerous viruses of their ability to cause disease and repurposing them as delivery trucks. The three most widely used are adeno-associated viruses (AAV), lentiviruses, and adenoviruses. Each has different strengths: AAV is small and triggers relatively mild immune reactions, making it popular for targeting specific organs like the eye or liver. Lentiviruses can integrate their cargo directly into a cell’s DNA, which makes them useful when you need the genetic change to be permanent and passed along as cells divide.
Non-viral delivery methods also exist, including tiny fat particles called lipid nanoparticles that can carry genetic material. These are the same basic technology behind mRNA COVID vaccines, though for gene therapy the payload and goals are different.
In Vivo vs. Ex Vivo: Two Routes of Treatment
Gene therapy reaches your cells through one of two paths, and they look very different from a patient’s perspective.
With in vivo gene therapy, the treatment goes directly into your body. This might be an IV infusion or, for eye diseases, an injection into the back of the eye. The viral vector circulates and enters target cells on its own. This approach works well when the cells you need to reach are hard to remove from the body, like brain cells or retinal cells.
Ex vivo gene therapy takes a longer, more involved route. Doctors first collect cells from your body, typically blood stem cells. Those cells go to a lab, where scientists use a viral vector to insert the corrected gene. The modified cells are grown, checked, and then infused back into you. This is the approach used for several blood disorders, and it requires significant preparation.
What CRISPR Changed
Traditional gene therapy mostly adds genes. CRISPR, which earned a Nobel Prize in 2020, lets scientists cut DNA at a precise location and then let the cell’s own repair machinery take over. The system uses a small piece of guide RNA to locate the exact stretch of DNA that needs changing, then a protein called Cas9 acts as molecular scissors to make the cut.
What happens next depends on how the cell repairs the break. The most common outcome is that the cell stitches the cut ends back together quickly but imperfectly, often scrambling a few letters of genetic code in the process. This is useful when you want to disable a gene: the scrambled sequence means the gene can no longer produce its protein. A more precise but less common repair pathway uses a template to rewrite the sequence at the cut site, allowing scientists to swap in the correct version of a gene. Researchers are actively engineering new versions of the Cas9 protein that shift the balance toward this more precise repair.
The first CRISPR-based gene therapy, Casgevy, received FDA approval for sickle cell disease and transfusion-dependent beta thalassemia, marking a turning point from laboratory tool to clinical treatment.
What the Treatment Process Looks Like
For in vivo gene therapies, the patient experience can be relatively simple. Luxturna, which treats an inherited form of blindness, involves a surgical injection into each eye. Zolgensma, for spinal muscular atrophy in young children, is a single IV infusion.
Ex vivo therapies are more demanding. Before your modified stem cells can be infused, your existing bone marrow typically needs to be cleared out through a conditioning regimen. This involves chemotherapy, and sometimes radiation, to suppress your immune system and make room for the new cells to grow. The process causes side effects common to chemotherapy: nausea, hair loss, fatigue, and increased infection risk. Children undergoing this kind of preparation often spend six to eight weeks in the hospital in a special room with purified air, and adults face a similar recovery window. The conditioning phase is often described by patients as the hardest part of gene therapy.
How Long the Effects Last
One of the biggest promises of gene therapy is that a single treatment could last a lifetime. The reality is more complicated and depends heavily on which organ is being treated.
A meta-analysis of 255 clinical trials using AAV vectors found that durability varies by tissue type. Therapies targeting the brain and nervous system maintained their effects in up to 90% of patients during follow-up. Muscle diseases held steady in about 73% of cases. But liver-targeted therapies maintained durable effects in only about 47% of trials, likely because liver cells divide frequently, gradually diluting the therapeutic gene over time. Eye therapies showed similar variability, with roughly 44% achieving lasting results.
Some trials have documented sustained benefits for three years or longer, but long-term data beyond a decade remains limited for most therapies. There’s also significant variability from patient to patient, influenced by age, genetic background, disease progression, and individual immune responses.
Immune Reactions and Safety Risks
Your immune system treats viral vectors the way it treats any foreign invader, and this creates the most significant safety challenge in gene therapy. Many people have already been exposed to natural AAV viruses during their lives and carry antibodies against them. These pre-existing antibodies can neutralize a gene therapy before it ever reaches target cells, which is why patients are screened for antibodies before treatment.
Even without pre-existing antibodies, the immune system mounts a response after treatment. Immune cells can identify and destroy cells that received the therapy, reducing its effectiveness. The body also creates memory cells that remember the viral vector, which means a second dose of the same therapy would trigger a faster, stronger reaction. This is a major reason most gene therapies are currently limited to a single dose.
Common reactions include inflammation and flu-like symptoms. At high doses, the immune response can become more serious, activating a cascade that leads to low platelet counts, bleeding problems, or injury to the liver, kidneys, or heart. These severe reactions are uncommon but have occurred in clinical trials, particularly with high-dose systemic infusions.
For gene-editing approaches like CRISPR, there’s an additional concern: off-target cuts. The guide RNA might direct Cas9 to a stretch of DNA that closely resembles, but isn’t, the intended target. An unintended cut in the wrong gene could theoretically cause new problems, though screening methods have improved substantially to minimize this risk.
Cost of Gene Therapy
Gene therapies are among the most expensive medical treatments ever developed. Zolgensma, for spinal muscular atrophy, launched at $2.125 million per dose and quickly set the benchmark. Nine gene therapies now cost more than $2 million, and the most expensive approved therapy runs $4.25 million for a single treatment. Luxturna, for inherited vision loss, costs $425,000 per eye.
Manufacturers argue these prices reflect the one-time nature of treatment compared to lifelong drug regimens, and that developing therapies for small patient populations requires recouping costs across fewer people. For patients and insurers, the financial burden remains a major barrier to access, even when the therapy itself works as intended.