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Frequent Questions

What is gene therapy?

In the broadest sense, gene therapy is the use of genetic material in the treatment or prevention of disease. The transferred genetic material changes how a single protein or group of proteins is produced by the cell. Gene therapy can be used to reduce levels of a disease-causing version of a protein, increase production of disease-fighting proteins, or to produce new/modified proteins.

What is cell therapy?

Cell therapy is the transfer of intact, live cells into a patient to help lessen or cure a disease. The cells may originate from the patient (autologous cells) or a donor (allogeneic cells). The cells used in cell therapy can be classified by their potential to transform into different cell types. Pluripotent cells can transform into any cell type in the body and multipotent cells can transform into other cell types, but their repertoire is more limited than that of pluripotent cells. Differentiated or primary cells are of a fixed type. The type of cells administered depends on the treatment.

What is the difference between gene therapy and cell therapy?

Gene therapy involves the transfer of genetic material, usually in a carrier or vector, and the uptake of the gene into the appropriate cells of the body. Cell therapy involves the transfer of cells with the relevant function into the patient. Some protocols utilize both gene therapy and cell therapy. In this case, stem cells are isolated from the patient, genetically modified in tissue culture to express a new gene, expanded to sufficient numbers, and then returned to the patient.

What is regenerative medicine?

Regenerative medicine focuses on the development of strategies to repair the functions of damaged organs or tissues. Recently, the American Medical Association has begun to use the term regenerative medicine for research and protocols involving stem cells in the repair of diseased tissue and organs. Two common approaches include the administration of stem cells for the regeneration of the indicated tissue or the administration of agents that enhance the patients resident tissue stem cells to more efficiently rebuild the damaged tissue. Recent advances have also been made in generating specific tissues and organs in the laboratory and safely implanting them into patients.

Why are stem cells so important in gene and cell therapy?

The goal of gene and cell therapy is to develop a treatment that lasts the lifetime of the patient. Most cells of the body turn over in days, weeks, or months. Changing the protein expression of a cell that lives only a few days, weeks, or months means that the therapy would require multiple administrations. A few cells, such as muscle cells, stem cells, neurons, and memory cells of the immune system, are long lived and may last the lifetime of the individual. Stem cells provide two major benefits for gene and cell therapy. First, they provide a cell type that can self-renew and may survive the lifetime of the patient. Second, stem cells provide daughter cells that mature into the specialized cells of each tissue. These differentiated daughter cells can replace the diseased cells of the afflicted tissue(s). Therefore, gene and cell therapy that uses stem cells theoretically improves the disease condition for as long as those modified stem cells live, potentially the lifetime of the patient. 

What kinds of diseases do gene and cell therapy treat?

Learn more from the Disease Treatments section of the ASGCT Patient Education program and ASGCT's Clinical Trials Finder.

Characteristics of diseases amenable to gene therapy and cell therapy include those for which there is no effective treatment, those with a known cause (such as a defective gene), those that have failed to improve or have become resistant to conventional therapy, and/or cases where current therapy involves long term administration of an expensive therapeutic agent or an invasive procedure. 

Gene therapy and cell therapy have the potential for high therapeutic gain for a broad range of diseases. An example would be those caused by a mutation in a single gene where an accessible tissue is available, such as bone marrow, and with the genetically modified cell ideally having a survival advantage. However, patients with similar symptoms may have mutations in different genes involved in the same biological process. For example, patients with hemophilia A have a mutation in blood clotting Factor VIII whereas patients with hemophilia B have a mutation in Factor IX. It is important to know which gene is mutated in a particular patient, as well as whether they produce an inactive protein which can help to avoid immune rejection of the normal protein. 

Gene therapy and cell therapy also offer a promising alternative or adjunct treatment for symptoms of many acquired diseases, such as cancer, rheumatoid arthritis, diabetes, Parkinson’s disease, Alzheimer’s disease, etc. Cancer is the most common disease in gene therapy clinical trials. Cancer gene therapy focuses on eliminating the cancer cells, blocking tumor vascularization and boosting the immune response to tumor antigens. Many gene and cell therapy approaches are being explored for the treatment of a variety of acquired diseases. More details are listed in the ASGCT.org disease information page.

Are there different types of gene therapy?


“ClinicalTrials.gov” lists more than 1000 different types of gene therapy in clinical trials. Additionally, almost any gene in the human genome can be targeted, so the potential for new therapies is immense. The five main therapeutic strategies are presented below. Currently, these techniques are mainly used to target specific populations of somatic cells.

Gene addition involves inserting a new copy of a gene into the target cells to produce more of a protein. Most often, a modified virus such as adeno-associated virus (AAV) is used to carry the gene into the cells. Therapies based on gene addition are being developed to treat many diseases, including adenosine deaminase severe combined immunodeficiency (ADA- SCID), congenital blindness, hemophilia, Leber’s congenital amaurosis, lysosomal storage diseases, X-linked chronic granulomatous disease, and many others.


Gene correction can be achieved by modifying part of a gene using recently-developed gene editing technology (e.g. CRISPR/cas9, TALEN or ZFN) to remove repeated or faulty elements of a gene, or to replace a damaged or dysfunctional region of DNA. The goal of gene correction is to produce a protein that functions in a normal manner instead of in a way that contributes to disease. It may be possible to use gene correction in the treatment of a wide range of diseases; recent experimental work has used gene editing technologies to extract HIV from the genome of affected laboratory mice and to excise the expanded region responsible for Huntington’s disease from the human gene.

Gene silencing prevents the production of a specific protein by targeting messenger RNA (mRNA; an intermediate required for protein expression from a gene) for degradation so that no protein is produced. mRNA exists in a single-stranded form in human and animal cells, whereas viruses have double-stranded RNA. Human and animal cells recognize double-stranded RNA as being viral in origin and destroy it to prevent its spread. Gene silencing uses small sequences of RNA to bind unique sequences in the target mRNA and make it double-stranded. This triggers the destruction of the mRNA using the cellular machinery that destroys viral RNA. Gene silencing is an appropriate gene therapy for the treatment of diseases where too much of a protein is produced. For example, too much tumor necrosis factor (TNF) alpha is often observed in the afflicted joints of rheumatoid arthritis patients. As TNF alpha is needed in small amounts in the rest of the body, gene silencing is used to reduce TNF alpha levels only in the affected tissue.

Reprogramming involves adding one or more genes to cells of a specific type to change the characteristics of those cells. This technique is particularly powerful in tissues where multiple cell types exist and the disease is caused by dysfunction in one type of cells. For example, type I diabetes occurs because many of the insulin-producing islet cells of the pancreas are damaged. At the same time, the cells of the pancreas that produce digestive enzymes are not damaged. Reprogramming these cells so that they can produce insulin would help heal type I diabetic patients.

Cell elimination strategies are typically used to destroy malignant (cancerous) tumor cells, but can also be used to target overgrowth of benign (non-cancerous) tumor cells. Tumor cells can be eliminated via the introduction of “suicide genes,” which enter the tumor cells and release a prodrug that induces cell death in those cells. Viruses can be engineered to have an affinity for tumor cells. These oncotropic viruses can carry therapeutic genes to increase toxicity to tumor cells, stimulate the immune system to attack the tumor, or inhibit the growth of blood vessels that supply the tumor with nutrients.

Are there different types of cell therapy?


"ClinicalTrials.gov” listed more than 8000 active, or actively recruiting, clinical trials for cell therapies being developed for diverse diseases.

The most common type of cell therapy is blood transfusion, and the transfusion of red blood cells, white blood cells, and platelets from a donor. Another common cell therapy is the transplantation of hematopoietic stem cells to create bone marrow which has been performed for over 40 years. As with gene therapy, cell therapy subtypes can be classified in different ways. This is currently no formal classification system for cell therapies. Here the different types of cells used for cell therapy have been classified by cell potency. Four types of pluripotent stem cells and four types of multipotent stem cells obtained from adult tissue are described.

Embryonic stem cells (ESCs). These are pluripotent stem cells derived from embryos. Generally, the embryos used to isolate stem cells are unused embryos generated from in vitro fertilization (IVF) for assisted reproduction. As ESCs are pluripotent they retain the ability to self-renew and to form any cell in the body. ESCs have the advantage of versatility due to their pluripotency, but the use of embryos in the development of therapeutic strategies raises some ethical concerns. In addition, stem cell lines generated from embryos are not genetically matched to the patient which can increase the chance that the transplanted cell is rejected by the patient’s immune system.


Induced pluripotent stem cells (iPSCs). A differentiated adult (somatic) cell, such as a skin cell is reprogrammed to return to a pluripotent state. These cells offer the advantage of pluripotency but without the ethical concerns of embryonic stem cells. iPSCs may also be derived from the patient and thus avoid the problem of immune rejection. iPSCs are produced by transforming the adult cell with a cocktail of genes usually delivered via a viral vector. While the efficiency of the process has been greatly improved since inception, the relatively low rate of reprogramming remains a concern. Another concern is that iPSCs are derived from adult cells and are therefore “older” than embryonic stem cells as evidenced by a higher rate of programmed cell death, lower rates of DNA damage repair and increased incidence of point mutations.


Nuclear transfer embryonic stem cells (ntESCs). These pluripotent cells are produced by transferring the nucleus from an adult cell obtained from the patient to an oocyte (egg cell) obtained from a donor. The process of transferring the nucleus reprograms the egg cell to pluripotency. As with iPSCs, the derived cells match the nuclear genome of the patient and are unlikely to be rejected by the body. However, the major advantage of this technique is that the resulting ntESCs carry the nuclear DNA of the patient alongside mitochondria from the donor, making this technique particularly appropriate for diseases where the mitochondria are damaged or dysfunctional. A drawback of ntESCs is that the process of generation is cumbersome and requires a donor oocyte. At the time of writing stem cell production using this technique has only been shown in lower mammals.

Parthenogenetic embryonic stem cells (pES). The final option for obtaining pluripotent cells is from unfertilized oocytes. Here the oocyte is treated with chemicals that induce embryo generation without the addition of sperm (parthenogenesis) and ESCs are harvested from the developing embryo. This technique generates ESCs that are genetically identical to the female patient. However, this method is in the early stages of development and it is not known if cells and tissues derived from parthenogenesis develop normally.

Hematopoietic stem cells (HSCs) are multipotent blood stem cells that give rise to all types of blood cells. HSCs can be found in adult bone marrow, peripheral blood, and umbilical cord blood.

Mesenchymal stem cells (MSCs) are multipotent cells present in multiple tissues including umbilical cord, bone marrow, and fat tissue. MSCs give rise to bone, cartilage, muscle, and adipocytes (fat cells) which promotes marrow adipose tissue.

Neural stem cells (NSCs). Adult neural stem cells are present in small number in defined regions of the mammalian brain. These multipotent cells replenish neurons and supporting cells of the brain. However, adult neural stem cells cannot be obtained from patients due to their location in the brain. Therefore, neural stem cells used for cell therapies are obtained from iPSCs or ESCs.

Epithelial stem cells. Epithelial cells are those that form the surfaces and linings of the body including the epidermis and the lining of the gastro-intestinal tract. Multipotent epithelial stem cells are found in these areas along with unipolar stem cells that only differentiate into one type of cell. Epithelial stem cells have been successfully used to regenerate the corneal epithelium of the eye.

Immune cell therapy. Cells that rapidly reproduce in the body such as immune cells, blood cells or skin cells can usually do so ex vivo given the right conditions. This allows differentiated, adult immune cells to be used for cell therapy. The cells can be removed from the body, isolated from a mixed cell population, modified and then expanded before return to the body. A recently developed cell therapy involves the transfer of adult self-renewing T lymphocytes which are genetically modified to increase their immune potency to kill disease-causing cells.

How are gene therapy and cell therapy related?

Both approaches have the potential to alleviate the underlying cause of genetic diseases and acquired diseases by replacing the missing protein(s) or cells causing the disease symptoms, suppressing expression of proteins which are toxic to cells, or eliminating cancerous cells.

Gene therapy involves the transfer of genetic material into the appropriate cells. In genetic diseases, the stem cells of the afflicted tissue are often targeted. The adult stem cells of the tissue can replenish the specialized cells. Expressing the appropriate gene in the stem cells ensures that the subsequent specialized cells will contain the therapeutic protein. However, in some cases, it’s technically easier to express a gene in a long-lived tissue cell and the secreted protein travels through the blood to its target organs. Introduction of genes into cells can be carried out in culture with subsequent administration to the patient, or by direct injection of vectors into the body.

Cell therapy is the transfer of cells to a patient. For treatment of most diseases by cell therapy, stem cells are chosen because their establishment in the patient leads to continual production of the appropriate specialized cells.

As mentioned previously, gene therapy and cell therapy are often combined to treat various genetic diseases, such as ADA-SCID. Stem cells from the patient are altered by gene therapy in culture to express the relevant functional protein. The improved stem cells are administered or returned to the patient.

How are viruses used in gene therapy?

Viruses are used in gene therapy as gene delivery vectors and as oncolytic viruses:

Viruses as gene delivery vectors. Modified viruses are used as carriers in gene therapy. These viral vectors protect the new gene from enzymes in the blood that can degrade it, and deliver it to the relevant cells. Viral vectors efficiently coerce the cells to take up the new gene, uncoat the gene from the virus particle, and transport it, usually to the cell nucleus. The transduced cells begin using the new gene to perform its function, such as synthesis of a new protein. Viral vectors are genetically engineered so that most of their essential genes are missing, which prevents uncontrolled replication of the virus and makes room for insertion of the gene to be delivered.

Many different viral vectors are being developed because the requirements of gene therapy agents for specific diseases vary depending on the affected tissue, the level of gene expression, and the required duration of expression. Scientists examine the following characteristics while choosing or developing an appropriate viral vector: (i) size of DNA or gene that can be packaged, (ii) efficiency of uptake by the desired cells for therapy, (iii) duration of gene expression, (iv) effect on immune response, (v) ease of manufacturing, (vi) ease of integration into the cell’s DNA or ability to exist as a stable DNA element in the cell nucleus without genomic integration, and (vii) chance that the patients have previously been exposed to the virus and thus might have antibodies against it which would reduce its efficiency of gene delivery.

Oncolytic Viruses. Oncolytic viruses are engineered to replicate only or predominantly in cancer cells and not in normal human cells. Once oncolytic viruses replicate in cancer cells they cause the cancer cells to burst, releasing more oncolytic viruses to infect surrounding cancer cells.

How does gene therapy work?

For a more detailed answer, we recommend Gene Therapy Basics in our Patient Education program.

Put simply, gene therapy works by changing the genetic information of a population of cells in a way that alleviates or combats the cause or symptoms of a disease.

What are some of the challenges gene and cell therapists face?

The challenges of gene and cell therapists can be divided into three broad categories based on disease, development of therapy, and funding.

Challenges based on the disease characteristics: Disease symptoms of most genetic diseases, such as Fabry’s, hemophilia, cystic fibrosis, muscular dystrophy, Huntington’s, and lysosomal storage diseases are caused by distinct mutations in single genes. Other diseases with a hereditary predisposition, such as Parkinson’s disease, Alzheimer’s disease, cancer, and dystonia may be caused by variations/mutations in several different genes combined with environmental causes. Note that there are many susceptible genes and additional mutations yet to be discovered. Gene replacement therapy for single gene defects is the most conceptually straightforward. However, even then the gene therapy agent may not equally reduce symptoms in patients with the same disease caused by different mutations, and even the same mutation can be associated with different degrees of disease severity. Gene therapists often screen their patients to determine the type of mutation causing the disease before enrollment into a clinical trial.

The mutated gene may cause symptoms in more than one cell type. Cystic fibrosis, for example, affects lung cells and the digestive tract, so the gene therapy agent may need to replace the defective gene or compensate for its consequences in more than one tissue for maximum benefit. Alternatively, cell therapy can utilize stem cells with the potential to mature into the multiple cell types to replace defective cells in different tissues.

In diseases like muscular dystrophy, for example, the high number of cells in muscles throughout the body that need to be corrected in order to substantially improve the symptoms makes delivery of genes and cells a challenging problem.


Some diseases, like cancer, are caused by mutations in multiple genes. Although different types of cancers have some common mutations, every tumor from a single type of cancer does not contain the same mutations. This phenomenon complicates the choice of a single gene therapy tactic and has led to the use of combination therapies and cell elimination strategies. For more information on gene and cell therapy strategies to treat cancer, please refer to the Cancer and Immunotherapy summary in the Disease Treatment section.

Disease models in animals do not completely mimic the human diseases and viral vectors may infect various species differently. The testing of vectors in animal models often resemble the responses obtained in humans, but the larger size of humans in comparison to rodents presents additional challenges in the efficiency of delivery and penetration of tissue. Gene therapy, cell therapy, and oligonucleotide-based therapy agents are often tested in larger animal models, including rabbit, dog, pig and nonhuman primate models. Testing human cell therapy in animal models is complicated by immune rejections. Furthermore, humans are a very heterogeneous population. Their immune responses to the vectors, altered cells, or cell therapy products may differ or be similar to results obtained in animal models.

Challenges in the development of gene and cell therapy agents: Scientific challenges include the development of gene therapy agents that express the gene in the relevant tissue at the appropriate level for the desired duration of time. There are a lot of issues in that once sentence, and while these issues are easy to state, each one requires extensive research to identify the best means of delivery, how to control sufficient levels or numbers of cells, and factors that influence duration of gene expression or cell survival. After the delivery modalities are determined, identification and engineering of a promoter and control elements (on/off switch and dimmer switch) that will produce the appropriate amount of protein in the target cell can be combined with the relevant gene. This “gene cassette” is engineered into a vector or introduced into the genome of a cell and the properties of the delivery vehicle are tested in different types of cells in tissue culture. Sometimes things go as planned and then studies can be moved onto examination in animal models. In most cases, the gene/cell therapy agent may need to be improved further by adding new control elements to obtain the desired responses in cells and animal models.

Furthermore, the response of the immune system needs to be considered based on the type of gene or cell therapy being undertaken. For example, in gene or cell therapy for cancer, one aim is to selectively boost the existing immune response to cancer cells. In contrast, to treat genetic diseases like hemophilia and cystic fibrosis the goal is for the therapeutic protein to be accepted as an addition to the patient’s immune system.

If the new gene is inserted into the patient’s cellular DNA, the intrinsic sequences surrounding the new gene can affect its expression and vice versa. Scientists are now examining short DNA segments that may insulate the new gene from surrounding control elements. Theoretically, these “insulator” sequences would also reduce the effect of vector control signals in the gene cassette on adjacent cellular genes. Studies are also focusing on means to target insertion of the new gene into “safe” areas of the genome, to avoid influence on surrounding genes and to reduce the risk of insertional mutagenesis.


Challenges of cell therapy include the harvesting of the appropriate cell populations and expansion or isolation of sufficient cells for one or multiple patients. Cell harvesting may require specific media to maintain the stem cells ability to self-renew and mature into the appropriate cells. Ideally “extra” cells are taken from the individual receiving therapy. Those additional cells can expand in culture and can be induced to become pluripotent stem cells (iPS), thus allowing them to assume a wide variety of cell types and avoiding immune rejection by the patient. The long term benefit of stem cell administration requires that the cells be introduced into the correct target tissue and become established functioning cells within the tissue. Several approaches are being investigated to increase the number of stem cells that become established in the relevant tissue.

Another challenge is developing methods that allow manipulation of the stem cells outside the body while maintaining the ability of those cells to produce more cells that mature into the desired specialized cell type. They need to provide the correct number of specialized cells and maintain their normal control of growth and cell division, otherwise there is the risk that these new cells may grow into tumors.

Challenges in funding: In most fields, funding for basic or applied research for gene and cell therapy is available through the National Institutes of Health (NIH) and private foundations. These are usually sufficient to cover the preclinical studies that suggest a potential benefit from a particular gene and cell therapy. Moving into clinical trials remains a huge challenge as it requires additional funding for manufacturing of clinical grade reagents, formal toxicology studies in animals, preparation of extensive regulatory documents, and costs of clinical trials. Biotechnology companies and the NIH are trying to meet the demand for this large expenditure, but many promising therapies are slowed down by lack of funding for this critical next phase.

What are stem cells and where do they come from?

Stem cells are cells that can self-renew and can mature into at least one type of specialized cell. Stem cells can be isolated from many types of tissues. Embryonic stem cells are isolated from the inner mass of the blastocyst, an early stage of the embryo. Umbilical cord stem cells, often called cord blood stem cells, are isolated from the umbilical cord at the time of a baby’s birth.

Adult stem cells can be isolated from any type of adult tissue. The ease of isolation of adult stem cells depends on the accessibility of the tissue, the prevalence of stem cells in the tissue, the age of the patient, the presence of markers that aid stem cell isolation, and developed protocols for isolation and culture. It is also possible to convert a mature adult cell into a stem cell by introducing a mixture of transcription factors; these cells are referred to as induced pluripotent stem (iPS) cells.

What are the differences between embryonic stem cells, adult stem cells, and iPS cells?

Embryonic stem cells are pluripotent stem cells isolated from an early stage embryo. They can self-renew and can differentiate into all cells of the body.

Adult stem cells are present in adult tissues. Each tissue has a reservoir of stem cells (sometimes called somatic stem cells). They can mature or differentiate into cells from that tissue. Adult stem cells can also be isolated from adipose tissue, gut, liver, brain, and muscle.

iPS stands for induced pluripotent stem cells. Specialized cells, such as skin cells, are isolated from adult tissues and treated with agents that change their protein expression pattern to mimic the proteins expressed by pluripotent stem cells. This process of reprogramming changes a cell with a specialized function to a cell with unlimited ability to self-renew and produce cells that can mature into all of the different types of specialized cells in the body. The process involves using gene delivery to express the relevant 3-4 genes that can convert the specialized cells into iPS cells.

What are the ethical issues associated with gene and cell therapy?

Several ethical issues can arise during the development of any novel therapeutic. The development of genetic and cellular therapies share many ethical issues with other types of therapy, such as prosthetics, drugs, organ transplantation, and protein replacement. In addition, there are ethical issues unique to gene and cell therapy. In all cases, scientists, clinicians, regulatory committees, and concerned citizens take an active role in addressing these issues.

Balancing risk and benefit to the patient is important to any developing therapeutic. This is complicated by the fact that most gene therapy trials are Phase I trials, which means that safety of the vector and delivery mode are being evaluated and no direct benefit to the participant is expected. To assess potential benefit, regulatory committees often request that investigators administer a range of doses of the agent for the initial patients to determine whether higher doses do have adverse effects—even during Phase II/III trials. Thus, the dosage tested in a particular patient may be insufficient to induce a therapeutic response or may be so high as to cause toxicity.

High costs associated with gene and cell therapy raise the ethical question of whether these treatments will only be utilized by the wealthy. Biotechnology companies such as Novartis, who developed the leukemia treatment Kymriah, are aware of this and are developing programs to provide financial help to patients in the USA who are uninsured or underinsured. Another factor to consider is that gene and cell therapies are designed to be curative, and so the cost of therapy can be weighed against that of lifetime treatment. In the long-term, costs will likely be reduced by optimized production of cell and gene therapies and the development of therapies that do not need to be tailored to the individual. In the meantime, patient groups, clinicians, regulators and manufacturers all have a role to play in addressing the issue of cost.

Contamination of the human genome with novel DNA sequences is a concern that may be considered in two ways. First, there is the issue of accidental contamination of the genome while conducting gene or cell therapy on somatic (adult) cells. To minimize the possibility of this, all vectors are tested to make sure they do not enter the germ line in experimental animals, and sperm from human males in clinical studies are tested to make sure the gene has not inserted in the genome. Second, there is the issue of intentional manipulation of the germline to alleviate disease. As new gene editing technologies have now made this much easier, there is currently much debate between scientists, clinicians, patient groups, and regulators regarding the ethics of editing, or not editing the human genome.

Use of embryonic stem cells, or human fetal tissue, as a source of stem cells remains an ethical issue. The development of stem cells from other sources such as iPSCs has somewhat reduced the dependence on ESCs.