How Gene Replacement Therapy Works

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Discover the Science of Gene Replacement Therapy

Learn how this approach attempts to address the root cause of genetic diseases.

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Gene replacement therapy is being studied as a treatment option for genetic diseases, and we want to help you understand the science behind it.

To begin, let’s talk about genes. Genes are small sections of DNA. DNA is inherited from your parents and carries instructions that tell the body how to function properly. Specific genes tell the body how to make specific proteins. Proteins play important roles in the body, like helping your cells to function properly or acting as the building blocks of your body. If a gene has an error, and the body can’t make a specific protein, it can be damaging to a person’s health.

A genetic disease or disorder is the result of an error in one or more of a person’s genes. The disease a person has depends on which gene in their DNA has the error. Depending on the disease or disorder, it can be inherited from one or both parents. Or, sometimes it’s a change that just happens randomly. A genetic disease caused by a single gene that is faulty or missing is called a monogenic disease.

We believe monogenic diseases are ideal targets for gene replacement therapy. Here’s why:

Gene replacement therapy is designed to target the root cause of a disease—the gene that doesn’t work properly—by delivering a new, working copy of the gene. The new gene carries the instructions for making the protein the body needs that it couldn’t make before. For example, the new gene may make a protein that is needed for cells to function properly. Once the protein is being made in those cells, there is the potential to stop disease progression. Time is critical because once cells lose their ability to function properly, they may not be able to be fixed, and any damage already done to the body may be irreversible.

So, how does gene replacement therapy work? In the lab, a new, working copy of a specific gene is made. Then, the new gene is put inside a delivery vehicle, called a vector. Vectors are chosen based on which type of cells in your body need the new gene. This is because vectors are very particular about the type of cell they choose to enter. For example, a certain vector may be used to deliver a gene to cells in the brain, while another vector may be used to deliver a gene to cells in the liver.

One type of vector is the adeno-associated virus, or AAV. AAVs are promising vectors because they are not known to make people sick. AAVs are able to travel through the body to target many different kinds of cells. When the AAV vector reaches the nucleus of the cell, it releases the working copy of the gene.

In this kind of gene replacement therapy, the new gene sits separate from your DNA, inside the nucleus, or control center, of the cell. The vector is then naturally broken down by the body, but the gene stays right where it was delivered. The gene starts working to make protein the body needs to work as it should.

Gene replacement therapy is a scientific advancement. For people who are living with monogenic diseases, it has the potential to create new treatment options and a new world of opportunities.

To learn more about gene replacement therapy, visit

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How does gene replacement therapy work?

A missing or nonworking gene and a new working copy of the gene

Gene replacement therapy starts with scientists creating a new, working copy of a missing or nonworking gene.

A gene within a vector (an envelope)

Then the new gene is placed inside a vector, which acts like an envelope and carries the gene to the right places throughout the body.

Adeno-associated virus, or AAV

A vector can be created by making changes to a naturally occurring virus. A virus is selected as a vector because of its ability to enter the body. One such virus, called an adeno-associated virus, or AAV, is used because it is not known to cause sickness in people.

A vector (an envelope) entering the body

Next, the vector is placed in the body and carries the new gene to the control center of the cells, also known as the nucleus.

A new gene

Once inside the nucleus, the new gene tells the body how to make the protein it needs. Finally, the rest of the vector is broken down by the body.

Check your understanding of gene replacement therapy


How does the new, working gene in gene replacement therapy get into the nucleus of the cell?

A vector carries a new, working copy of a gene through the body and into the nucleus of a cell. Once the gene is in the nucleus, the vector is broken down by the body.

Learn more about genes and vectors.