Gene therapy is the insertion of genes into an individual's cells and tissues to treat a disease, and hereditary diseases in which a defective mutant allele is replaced with a functional one.
Although the technology is still in its infancy, it has been used with some success.
Antisense therapy is not strictly a form of gene therapy, but is a genetically-mediated therapy and is often considered together with other methods.
In most gene therapy studies, a "normal" gene is inserted into the genome to replace an "abnormal," disease-causing gene.
A carrier called a vector must be used to deliver the therapeutic gene to the patient's target cells.
Currently, the most common type of vectors are viruses that have been genetically altered to carry normal human DNA.
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.
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.
In theory it is possible to transform either somatic cells (most cells of the body) or cells of the germline (such as sperm cells, ova, and their stem cell precursors).
All gene therapy to date on humans has been directed at somatic cells, whereas germline engineering in humans remains controversial.
For the introduced gene to be transmitted normally to offspring, it needs not only to be inserted into the cell, but also to be incorporated into the chromosomes by genetic recombination.
Somatic gene therapy can be broadly split in to two categories: ex vivo, which means exterior (where cells are modified outside the body and then transplanted back in again) and in vivo, which means interior (where genes are changed in cells still in the body).
Recombination-based approaches in vivo are especially uncommon, because for most DNA constructs recombination has a very low probability.