Frequently Asked Questions

There’s a lot to learn when considering participating in an investigational gene therapy clinical trial. Start with the genes, and then continue on to learn about gene therapy research and clinical trials. Click on each category for more information.

About genes and genetic conditions

About cells:

  • Our bodies are made up of many types of cells, such as muscle and nerve cells.
  • As we grow, cells divide to make more cells to help take care of everything in our growing/changing body, from fixing a skinned elbow to replacing old or damaged cells.
  • Cells shape the body, which is why we are composed of trillions of them. From converting food into energy to carrying out specific functions in the body, cells help to keep everything running properly.
  • Cells contain genetic information, or DNA.

About DNA and genes:

  • Inside almost every cell, you will find DNA (also known as deoxyribonucleic acid).
  • Genes are made up of DNA.
  • DNA in the genes contains instructions for the body to make proteins needed for a person to grow, develop, and survive.

Genes are the body’s instruction manual. The human body is made up of trillions of cells. At the center of each cell is the nucleus, which contains all of the instructions the body needs to function.

Learn about how these instructions are stored on chromosomes, made of DNA.

To understand the science behind gene therapy research, we first need to understand the role genes play in the body.

Watch the Video: Discover the Role Genes Play in Hemophilia.

Genetic conditions are caused by a change, or mutation, in genes, where proteins are not made correctly.

Mutated genes can cause a genetic condition if they:

  • Make a protein that doesn’t work correctly.
  • Do not make enough or any of the needed protein.

To function properly, the body needs the right proteins to do the right job at the right time.

In hemophilia, there is a mutation in the gene that contains the body’s instructions for creating one of several important blood-clotting proteins.

Learn more about the role genes play in hemophilia.

About gene therapy

  • Gene therapy seeks to introduce new, functional genetic material into a person’s body, with the goal of treating, preventing, or potentially curing a disease.
  • Gene therapies have been researched in rare genetic diseases for over 50 years and take many years to develop.
  • So far, only a few gene therapies have been approved for use outside of a clinical trial.
  • Most gene therapies are still in the research phase, and there are hundreds of ongoing clinical trials of gene therapy for the treatment of rare genetic diseases.

What is investigational gene therapy meant to do?

Review a step-by-step overview.

If you’re interested in learning more about gene therapy clinical trials for hemophilia, speak with your doctor and visit

  • Some gene therapy techniques work by replacing, changing, or inactivating a gene that is causing a disease.
    • Investigational gene therapy aims to address a genetic condition at its source—the gene.
    • Gene therapy is intended to treat the root cause of genetic diseases by correcting problems with how genes make proteins.
    • Depending on the disease, gene therapy can be used to correct issues caused by too much, too little, or faulty protein being made.
  • One gene therapy approach uses viral vectors, which act as a delivery vehicle to bring the new gene into a target cell.
    • A viral vector is made from the shell of a virus, meaning all the viral DNA has been removed so that it can’t make people sick.
    • There are many types of viral vectors used for gene therapy; each one has a particular attraction to certain types of cells.

The adeno-associated virus (AAV) is commonly used as a vector in gene therapy research overall, and specifically in gene therapy research for hemophilia.

Learn more about AAV gene therapy for hemophilia.

Scientists are studying a variety of approaches for treating or preventing genetic disease at its source—the gene. Here are 3 approaches to gene therapy using AAV vectors.

Illustrations showing the process of gene transfer therapy.

Gene transfer therapy, the main gene therapy approach discussed on this site, which is also referred to as gene addition, creates new functional genes and delivers them to specific cells in the body with an aim to restore or enhance normal function. Currently, gene transfer therapy is the approach being most broadly investigated in hemophilia.

Illustrations showing the process of gene editing.

Gene editing, sometimes called genome editing or genomic editing, modifies existing genes in a person’s DNA to correct specific mutations. Some examples of investigational gene-editing technologies include CRISPR and zinc-finger nucleases.

Illustrations showing the process of cell therapy.

Cell therapy transplants whole cells capable of adding a new or desired function into a person. The genes in the cells may or may not be modified by gene therapy or gene editing while outside the body before being returned to the patient.

Adeno-associated virus (AAV) is commonly used as a vector in gene therapy research overall, and specifically in gene therapy research for hemophilia.

Learn more about AAV gene therapy for hemophilia.

The primary research goal is to evaluate the safety and effectiveness of gene therapy intended to deliver a functional gene. Investigational gene therapy for hemophilia is designed to add a functional copy of the factor VIII or factor IX gene to a cell’s command center (nucleus). If transferred successfully, the functional gene is intended to provide the cell with the instructions to make factor VIII or factor IX clotting protein.

Learn more about the process.

About clinical trials

The safety and well-being of patients are top priorities.

Following many years of initial (preclinical) research, which includes laboratory, manufacturing, and animal studies, investigational new therapies may then be reviewed and authorized by the FDA for research studies in humans, known as clinical trials.

Learn more about the phases and extensive reviews of clinical trials.

Informed consent is a two-way conversation between you and a clinical trial’s research team. It includes the process of giving you all the facts about the trial, obligations of participation, and known and potential safety risks.

Learn more about informed consent.

Knowledge is power. Talk with your healthcare team to discuss any questions you have. Make sure you understand the road map for your gene therapy experience—everything before, the day of, and after—including any potential lifestyle changes.

Learn more about what to expect in a clinical trial.

Before beginning a clinical trial, potential participants are screened to determine who is eligible (inclusion criteria) or ineligible (exclusion criteria). Because clinical trials differ, they typically have different screenings. That means if you are not eligible for one clinical trial, you may still be eligible for another.

Learn more about potential screenings.

Potential risks associated with gene therapy for hemophilia include reactions related to the immune system and liver, which may require treatment with steroids. In addition, there is a theoretical increased risk of cancer as a result of the vector inserting itself into the DNA of cells in the body. To track the occurrence of these risks, a hemophilia gene therapy is studied in clinical trials under controlled conditions. While the safety of investigational gene therapy is still being studied, research to date has helped scientists learn important lessons.

Learn more about the potential risks.

  • The scientific community continues to research gene therapy for hemophilia despite the long timeline. This research is dedicated to working toward a transition in the treatment experience. The potential of a single or infrequent intervention with continuously expressed level versus recurrent treatment interventions with highs and lows (peaks and troughs) in factor level is the ultimate goal.
  • For hemophilia specifically, as the factor level comes down, the risk of a bleeding episode increases. Gene therapy research in hemophilia is aiming for patients to be kept above some threshold level of factor so there is a higher possibility of potentially preventing spontaneous bleeding episodes. The safety and efficacy of investigational gene therapy is still being explored.

Learn more about what to expect in a clinical trial.

Yes. Investigational gene therapy for hemophilia does not change your body’s DNA. You will still have hemophilia, and it could still be passed down to your children. However, your experience living with hemophilia could be different, as gene therapy is designed to help your body increase the amount of factor produced in the body.

Learn more about factors and the genetics of hemophilia.

Learning more

If you’re interested in learning more about gene therapy clinical trials for hemophilia, speak with your physician and visit

Every clinical trial is different, so you should talk with your doctor and healthcare team if you are considering participating in a clinical trial so that you understand everything it entails. Each trial has its own unique criteria, which will be covered with you during the informed consent process, but here’s an overview of what to expect:

  • Before dosing day, there will be an observational period.
  • On dosing day, your doctor and clinical trial research team will discuss the logistics of your infusion day with you.
    • The infusion typically takes about an hour.
    • You may be required to stay at the study site overnight or return the next day for some tests.
  • There will be defined time frames for follow-ups after dosing day for continued monitoring. The follow-up is critical for the study team to fully understand the safety and effectiveness of a gene therapy. What’s being measured in these follow-ups?

See more about what to expect.

Learning more

If you’re interested in learning more about gene therapy clinical trials for hemophilia, speak with your physician and visit

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