• Replacing a mutated gene that causes disease with a healthy copy of the gene
• Inactivating, or “knocking out,” a mutated gene that is functioning improperly
• Introducing a new gene into the body to help fight a disease

In general, a gene cannot be directly inserted into a person’s cell. It must be delivered to the cell using a carrier, or vector. Vector systems can be divided into:

• Viral vectors
• Non-viral vectors

Currently, the most common type of vectors are viruses that have been genetically altered to carry normal human DNA (see also Wiley database on vectors used in gene therapy trials). Viruses have evolved a way of encapsulating and delivering their genes to human cells in a pathogenic manner. Scientists have tried to harness this ability by manipulating the viral genome to remove disease-causing genes and insert therapeutic ones.

In theory it is possible to transform either somatic cells (most cells of the body) or cells of the germline (eggs or sperm). The distinction is that the results of any somatic gene therapy are restricted to the actual patient and are not passed on to his or her children. All gene therapy to date on humans has been directed at somatic cells, whereas germline engineering in humans remains controversial and prohibited in for instance the European Union.

Somatic gene therapy can be broadly split into two categories:

• ex vivo, which means exterior (where cells are modified outside the body and then transplanted back in again). In some gene therapy clinical trials, cells from the patient’s blood or bone marrow are removed and grown in the laboratory. The cells are exposed to the virus that is carrying the desired gene. The virus enters the cells and inserts the desired gene into the cells’ DNA. The cells grow in the laboratory and are then returned to the patient by injection into a vein. This type of gene therapy is called ex vivo because the cells are treated outside the body.
• in vivo, which means interior (where genes are changed in cells still in the body). This form of gene therapy is called in vivo, because the gene is transferred to cells inside the patient’s body.

- Basic process of gene therapy
- Types of gene therapy
- Viral vectors
   - Adenoviruses
   - Adeno-associated viruses
   - Lentiviruses
   - Retroviruses
- Non-viral vectors
   - Naked DNA
   - Oligonucleotides
   - Lipoplexes and polyplexes
- Glossary of gene therapy terms