What is gene therapy?

Gene therapy is a treatment that changes something in your own cells, so that your body can make healthy red blood cells. It is not a tablet you take or an operation. It is done once, over several months, using your own stem cells.

In the UK it is used for two conditions: sickle cell disease and transfusion-dependent beta thalassaemia. Both are caused by a change in the instructions your body uses to make haemoglobin, the part of a red blood cell that carries oxygen.

The short version

Doctors collect stem cells from your blood. In a laboratory, those cells are changed so they can make working haemoglobin. Meanwhile you have strong chemotherapy, which clears out the bone marrow you already have. Then the changed cells are given back to you through a drip, and they slowly rebuild your blood.

Because the cells are your own, there is no donor and no risk of your body rejecting them. But the chemotherapy is serious, and it is the part with the most lasting consequences.

Two different approaches

You may hear gene therapy talked about as if it were one thing. There are actually two quite different approaches, and knowing which one is being discussed makes a lot of the rest easier to follow.

Gene editing

This approach does not repair the faulty haemoglobin gene at all. Instead it switches a different gene back on. Before you were born, your body made a type of haemoglobin called fetal haemoglobin, and then switched it off. Gene editing disrupts the switch that turned it off, so your body starts making fetal haemoglobin again. That healthy haemoglobin does the job the faulty adult version cannot.

This is what Casgevy does. It uses CRISPR, a tool that makes a precise cut in DNA at one specific place. It is the only gene therapy for these conditions licensed in the UK.

Gene addition

This approach adds a working copy of the haemoglobin gene into your cells, delivered by a modified virus that has been stripped of its ability to cause illness. The virus is only a delivery vehicle. Two products work this way, Lyfgenia and Zynteglo, but neither is licensed for use in the UK.

What actually happens to you

The steps are the same whichever approach is used, and they take months rather than weeks.

1. Collecting your stem cells. You have medicines that push stem cells out of your bone marrow into your bloodstream, then the cells are collected from your blood by a machine. This can need to happen more than once.

2. Waiting while your cells are changed. Your cells go to a laboratory to be edited or modified, then frozen until you are ready.

3. Conditioning chemotherapy. High-dose chemotherapy clears the bone marrow to make room for the new cells. This is the hardest part of the treatment for most people.

4. The infusion. The changed cells are given back through a drip. It is a single dose and takes a short time.

5. Engraftment and recovery. The cells settle into your bone marrow and start making blood. Until they do, you have very little immune protection.

NHS England says patients stay in hospital for four to six weeks while the modified cells take hold. That figure comes from its announcement about thalassaemia. In the trials the inpatient stay was often longer, a median of 47 days, and up to about four months for some people. Recovery then continues well beyond going home.

What the trials showed

When the MHRA licensed Casgevy in November 2023, it reported the results the decision was based on.

• Sickle cell disease: of 29 patients assessed, 28 (97%) had no severe pain crises for at least 12 months after treatment.

• Beta thalassaemia: of 42 patients assessed, 39 (93%) needed no red blood cell transfusions for at least 12 months. The other three needed more than 70% fewer.

Those are genuinely striking numbers, and they are also small studies with no comparison group. NHS England has described the results as a 'functional cure' for most people treated. NICE's own committee, looking at the same evidence, said the treatment 'was not compared with anything else, the number of people in the trial was small', and that long-term effectiveness is still unclear. Both of those statements are true at the same time, and it is worth knowing that before you read anything more confident elsewhere.

The risks

Most of the serious risks come from the conditioning chemotherapy rather than from the gene therapy itself.

Fertility

The chemotherapy used before gene therapy can cause infertility. Regulators state plainly that it may prevent future pregnancy or fatherhood, and that fertility preservation options should be discussed before treatment starts. This matters for timing: those decisions have to be made before conditioning begins, not afterwards.

There are no data on what the gene therapy product itself does to fertility, in humans or in animals. The known risk comes from the chemotherapy; the product's own effect is simply unstudied.

During and after treatment

• While your immune system is rebuilding you are at high risk of infection, and this is why the hospital stay is long.

• Platelets, the part of blood that helps clotting, can take a long time to recover, which means a risk of bleeding.

• The new cells can fail to take hold. A back-up sample of your own untreated cells is always collected and kept in case it is needed.

• Some people react to the preservative used to freeze the cells.

Cancer risk differs between the two approaches

This is one of the clearest differences between gene addition and gene editing, and it is worth understanding rather than glossing.

Lyfgenia, which adds a gene using a viral vector, carries a boxed warning, the strongest warning a US regulator issues, because blood cancer has occurred in patients who received it. People treated with it need blood counts at least every six months, long term.

Casgevy, the gene editing treatment, does not carry that warning. The European Medicines Agency describes a cancer risk from unintended genetic changes as theoretical, with none seen so far. The US regulator added a warning in 2025 that off-target editing, the tool cutting somewhere it was not meant to, cannot be ruled out. Nothing has been observed; it has not been excluded either.

What isn't known yet

These treatments are recent, and honest information about them includes what nobody can tell you yet.

• Whether the benefit lasts for decades. The longest follow-up is a few years. Patients are asked to join a registry and be followed for 15 years, precisely because the long-term picture is unknown.

• How often rarer complications happen. The trials were too small to show this.

• Whether off-target edits cause harm over a lifetime.

• How well it works in younger children. The largest published study in children aged 5 to 11 included 26 children with a median follow-up of about 16 months. Every one of those children had at least one severe or life-threatening side effect during the process.

The European approval is conditional, meaning the company must keep supplying evidence. NHS funding in England is through a managed access agreement, which exists specifically to gather data over five years while these questions are still open. That is not a formality, it is the system saying openly that it does not yet know everything it wants to know.

Can you get it on the NHS?

In England, yes, for some people, through managed access rather than as a routine treatment.

• For transfusion-dependent beta thalassaemia, NICE recommended it in September 2024 for people aged 12 and over. NHS England estimated around 460 people in England might be eligible.

• For severe sickle cell disease, NICE recommended it in February 2025 for people aged 12 and over who have had at least two severe pain crises a year for the past two years. NHS England expected around 50 people a year to be treated, out of roughly 15,000 people in England living with sickle cell disease.

In both cases the licence applies where a stem cell transplant would be suitable but there is no matched related donor available. The treatment is given at a small number of specialist centres, not at every hospital.

The numbers above are for England. Arrangements in Scotland, Wales and Northern Ireland are decided separately and are not covered here. Whether gene therapy is an option for you personally is a question only your own haematology team can answer.

Questions you might take to your team

You do not have to arrive at an appointment knowing what to ask. These are questions other people have found useful.

• Am I likely to be eligible, and if not now, could that change?

• What would you expect the chemotherapy to be like for me specifically?

• What are my options for preserving fertility, and by when would I need to decide?

• What happens if the cells do not take hold?

• How long would I be in hospital, and how far would I have to travel?

• What would follow-up look like over the next five, ten, fifteen years?

• What are the alternatives if I decide not to do this, or not to do it yet?