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Imagine that you accidentally broke one of your neighbor's windows. What would you do? You could:

  1. Stay silent: no one will ever find out that you are guilty, but the window doesn't get fixed.
  2. Try to repair the cracked window with some tape: not the best long-term solution.
  3. Put in a new window: not only do you solve the problem, but also you do the honorable thing.

What does this have to do with gene therapy?

You can think of a medical condition or illness as a "broken window." Many medical conditions result from flaws, or mutations, in one or more of a person's genes. Mutations cause the protein encoded by that gene to malfunction. When a protein malfunctions, cells that rely on that protein's function can't behave normally, causing problems for whole tissues or organs. Medical conditions related to gene mutations are called genetic disorders.

So, if a flawed gene caused our "broken window," can you "fix" it? What are your options?

  1. Stay silent: ignore the genetic disorder and nothing gets fixed.
  2. Try to treat the disorder with drugs or other approaches: depending on the disorder, treatment may or may not be a good long-term solution.
  3. Put in a normal, functioning copy of the gene: if you can do this, it may solve the problem!

If it is successful, gene therapy provides a way to fix a problem at its source. Adding a corrected copy of the gene may help the affected cells, tissues and organs work properly. Gene therapy differs from traditional drug-based approaches, which may treat the problem, but which do not repair the underlying genetic flaw.

But gene therapy is not a simple solution - it's not a molecular bandage that will automatically fix a disorder. Although scientists and physicians have made progress in gene therapy research, they have much more work to do before they can realize its full potential. In this module, you'll explore several approaches to gene therapy, try them out yourself, and figure out why creating successful gene-based therapies is so challenging.


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Explore the methods for delivering genes into cells.

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You are the doctor! Design and test gene therapy treatments with ailing aliens.



Gene therapy could potentially treat certain disorders at the source by repairing the underlying genetic flaws. Many disorders or medical conditions might be treated using gene therapy, but others may not be suitable for this approach.

How do you know whether a disorder is a good candidate for gene therapy?

For any candidate disorder, you need to answer the following questions:

  1. Does the condition result from mutations in one or more genes? For you to even consider gene therapy, the answer must be "yes."
  2. Which genes are involved? If you plan to treat a genetic flaw, you need to know which gene(s) to pursue. You must also have a DNA copy of that gene available in your laboratory. The best candidates for gene therapy are the so-called "single-gene" disorders - which are caused by mutations in only one gene.
  3. What do you know about the biology of the disorder? To design the best possible approach, you need to learn all you can about how the gene factors into the disorder. For example:

    • Which tissues are affected?
    • What role does the protein encoded by the gene play within the cells of that tissue?
    • Exactly how do mutations in the gene affect the protein's function?
  4. Will adding a normal copy of the gene fix the problem in the affected tissue? This may seem like an obvious question, but it's not. What if the mutated gene encodes a protein that prevents the normal protein from doing its job? Mutated genes that function this way are called dominant negative and adding back the normal protein won't fix the problem. Learn more about how researchers are trying to address dominant negative mutated genes in New Approaches to Gene Therapy.
  5. Can you deliver the gene to cells of the affected tissue? The answer will come from several pieces of information, including:
    • How accessible is the tissue? Is it fairly easy (skin, blood or lungs), or more difficult to reach (internal organs)?
    • What is your best mode of delivery? You can examine the pros and cons of potential delivery methods in Tools of the Trade.

If you can answer "yes" to Questions 4 and 5, then the disorder may be a good candidate for a gene therapy approach.



In Choosing Targets for Gene Therapy, we saw that cystic fibrosis is a good candidate for gene therapy. This is because:

  • We know which gene is mutated in the disorder.
  • We have a normal copy of that gene available.
  • We understand the biology of the disease, including which tissue types are affected and how they are affected.
  • We can predict that adding the normal gene back to the cells that make up the affected tissues will restore a needed function.

Now all we have to do is deliver the gene into the proper cells and put it to work. This is not an easy job. Gene delivery is one of the biggest challenges in the field of gene therapy.

What are some of the hallmarks of successful gene delivery?

Target 1. TARGETING the right cells. If you want to deliver a gene into cells of the liver, it shouldn't wind up in the big toe. How can you ensure that the gene gets into the correct cells?

Activate 2. ACTIVATING the gene. A gene's journey is not over when it enters the cell. It must go to the cell's nucleus and be "turned on," meaning that its transcription and translation are activated to produce the protein product encoded by the gene. For gene delivery to be successful, the protein that is produced must function properly.

Integrate 3. INTEGRATING the gene in the cells. You might want the gene to stay put and continue working in the target cells. If so, you need to ensure that the gene integrates into, or becomes part of the host cell's genetic material, or that the gene finds another way to survive in the nucleus without being trashed.

Avoid 4. AVOIDING harmful side effects. Anytime you introduce an unfamiliar biological substance into the body, there is a risk that it will be toxic or that the body will mount an immune response against it. If the body develops immunity against a specific gene delivery vehicle, future rounds of the therapy will be ineffective.

Explore some of the gene delivery methods that researchers have developed in Tools of the Trade.

NCRR/SEPA Supported by a Science Education Partnership Award (SEPA) [No. 1 R25 RR16291-01] from the National Center for Research Resources, a component of the National Institutes of Health, Department of Health and Human Services. The contents provided here are solely the responsibility of the authors and do not necessarily represent the official views of NCRR or NIH.





Additional Resources
The Latest News on Gene Therapy Research
General Information about Gene Therapy Research
Regulation of Gene Therapy Research
Positions on Gene Therapy Research
The Jesse Gelsinger Story
Severe Combined Immune Deficiency
Gene Therapy for Adenosime Deaminase (ADA) Deficiency:
Issues and Ethics



                                                                      Capecchi's Transgenic Technology "Knocks Out" the Nobel Prize;

Mario R. Capecchi, Ph.D., of the University of Utah, has won the 2007 Nobel Prize in Physiology or Medicine. Capecchi shares the prize with Oliver Smithies of University of North Carolina, Chapel Hill and Sir Martin Evans of Cardiff University in the UK.

The prize recognizes Capecchi's pioneering work on "knockout mouse" technology, a gene-targeting technique that has revolutionized genetic and biomedical research, allowing the creation of animal models for hundreds of human diseases.

Mario Capecchi Portrait What's the Motivation?

During the 1980s, Capecchi devised a way to change or remove any single gene in the mouse genome, creating strains of mice that pass the altered gene from parent to offspring. In the years since, these "transgenic" and "knockout" mice have become commonplace in the laboratory.

Capecchi's pioneering work in gene targeting has taught us much about how the body builds - and rebuilds - itself. He has given scientists worldwide the tools to make important discoveries about human diseases from cancer to obesity.

And he has raised a key question for the future of human medicine: if we can replace a perfectly good gene with a mutated one, can we also go the other way, replacing problem genes with those that work?

"I headed south,sometimes living in the streets, sometimes joining gangs of other homeless children, sometimes living in orphanages and most of the time hungry." -Mario Capecchi

As a child, he wandered homeless in Italy. As a researcher, his first attempts at gene targeting were deemed "not worthy of pursuit" by the National Institutes of Health. Capecchi is an individual whose personal life proves that, while some events are not probable, anything is possible. Read Mario's story.

Mario's Lab at Work

What makes an arm an arm? Capecchi's research team is working on answering that question using gene targeting. They have systematically "knocked out" a set of genes in mice, called homeotic genes, which govern body patterning during development. For example, one of the lab's most recent genetic discoveries may explain why we lack spare ribs. Find out more about how homeotic genes work in Genes Determine Body Patterns.

Mouse Jar How to Build a "Knockout" Mouse

YOUR GOAL: You are studying how a particular gene, named OhNo, might play a role in panic attacks. You want to study mice with this gene turned off. To "knockout" the OhNo gene you will replace it with a mutated copy that doesn't work. Here's how:

Normal Stem Cells

Isolate embryonic stem cells that originated from male brown mice with a normal OhNo gene (blue).

Add Inactive Gene With Marker

To these cells, add a copy containing a mutated, inactive OhNo gene (red), and a drug resistance marker gene (pink).

Similar Genes Naturally Swap

By mechanisms that are not completely understood yet, similar genes will swap places. The mutant OhNo gene plus drug resistance marker gene is now incorporated into the genome and the normal version is kicked out. This is called homologous recombination.

Cells that haven't incorporated the inactive OhNo gene don't have the drug resistance marker gene (pink).

Adding the drug kills cells without the marker, leaving you with a clean batch of cells that all have an inactive version of the OhNo gene.

By transplanting stem cells that carry the inactive ohNo gene into a white mouse embyro, you'll create what is called a chimera. Chimeras have patches of cells throughout their bodies that grew from white mouse cells and patches that grew from brown stem cells. Some of the cells that have the inactive OhNo gene may develop into reproductive cells.
Chimeras are easy to identify because they have both brown and white patches of fur.

Mate Male Chimera

If a male chimera has some reproductive cells (sperm) that originated from the brown stem cells, he will produce some brown offspring when mated with a white female.


Half of the brown offspring will have a copy of the inactive OhNo gene in all of their cells - including their reproductive cells. These mice have one normal copy of the OhNo gene (not shown) and one inactive copy. So half of their reproductive cells will contain a normal copy and half will contain an inactive copy.


These mice can be identified by PCR and then bred with each other.

You've Made a "Knockout" Mouse!

One fourth of your resulting offsping will have two copies of the "knocked-out" or inactive OhNo genes. You can now study these mice to determine how lacking the OhNo gene may affect panic attacks.