- 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:

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 (see also video 2).

Target cells such as the patient's liver or lung cells are infected with the vector. The vector then unloads its genetic material containing the therapeutic human gene into the target cell. The generation of a functional protein product from the therapeutic gene restores the target cell to a normal state.