Gene therapy is a way of inserting genes into a patient's cells and replacing the preexisting alleles, or gene variants, to perform some therapeutic function. It has been used thus far primarily to replace mutant defective genes, or alleles, with normal alleles, but could in theory be used to edit the human genome arbitrarily. If gene therapy were applied to reproductive cells in the gonads (the germline), these genetic changes would be heritable. This process has never been performed, but it has a name: germline genetic engineering.
Since the early 1980s, gene therapy has been used to produce medicines. Say that a human being needs a certain protein as a medicine. This therapy uses a viral vector, that is, a virus modified to contain the DNA to be introduced. Large quantities of the virus are injected to the target area, or, sometimes tissue is removed, infected with the virus, and then implanted again. The viruses are modified such that the vast majority are not capable of independent self-replication - providing little chance for pathogenic infection. The virus introduced the new DNA into the genome of human cells, much in the same way normal viruses introduce their own genetic material into human cells, hijacking the cellular machinery.
After the new DNA is integrated into the target cell, the cell begins to manufacture proteins specified by the new genetic material, which in some instances, can be lifesaving. For example, patients with severe diabetes may be given the cellular machinery to produce insulin, obviating the need for regular injections. The benefits of the therapy can last for weeks, months, or even years or a lifetime.
Gene therapy has been used successfully to treat inherited retinal disease, thalassaemia, cystic fibrosis, severe combined immunodeficiency, and some cancers. Medical miracles not possible with any other approach have been demonstrated by gene therapy, such as reprogramming the body's natural sentinels, T-cells, to attack cancer cells. Gene therapy shows promise for treating afflictions such as Huntington's disease and sickle cell anemia. As the therapy continues to mature, it could save millions of lives.