How are genes transferred into cells so that gene therapy can take place?

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.” The vectors most commonly used in gene therapy are viruses. Viruses have a unique ability to recognize certain cells and insert genetic material into them.

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 grown outside the body. The gene is transferred into the patient's cells while the cells are outside the patient's body.

In other studies, vectors or liposomes are used to deliver the desired gene to cells in the patient's body. This form of gene therapy is called in vivo, because the gene is transferred to cells inside the patient's body.