One of the major variations in gene therapy is the choice of a vector. A vector is a carrier that brings the genetic material to the targeted cell. Vectors must conform to certain requirements in order to be effective in gene therapy. For example, they must be small enough to enter tissues and cells, and they must be able to deliver the genetic material to other parts of the body.

Vectors can be viral or non-viral. Viral vectors contain viruses, which are parasitic packages of genetic material encased in protein. Some viruses may be harmless to humans, but many cause infectious diseases, ranging from colds and flu to HIV/AIDS. Viruses used in gene therapy are “gutless,” which means that the harmful material has been removed. Viruses are used because they “infect” the body and are able to replicate whatever genetic material is placed inside them.

However, researchers have raised concerns about the safety of viral vectors, so scientists are working to create non-viral vectors. For example, fats (lipids) are being tested for potential effectiveness as a vector. There are numerous types of lipids available. Liposomes are synthetic fatty shells created to carry genetic material or medicine. They are not infectious but are small and unstable, which currently limits research potential. Additionally, lipoplexes are made of a liposome and DNA. They are larger and can carry more genetic material than a liposome. Scientists are also researching the use of proteins, transposons (segments of DNA that can transfer genetic material), plasmids (DNA molecules derived from bacteria) and ultrathin polymer films as nonviral vectors.

Another variation in gene therapy involves the way in which gene therapy is delivered. In some cases, treatment is in vivo (within the body). During in vivo treatment, the modified genes are introduced directly inside the body. Conversely, ex vivo treatment (outside the body) involves genetic cells being extracted from the body, modified in a laboratory and then re-introduced to the body. Both types have advantages and disadvantages: In vivo is an easier technological method, but ex vivo offers more control over undesirable factors that may develop before the gene is introduced into a person.