One potential benefit of the Human Genome Project will be the further refinement of gene therapy. When all of our genes and their functions are known, we will have a blueprint that tells us what genes, and what mutations of genes, are responsible for a vast array of human diseases. Gene therapy is intended to stop many of those diseases in their tracks, at their source.

If a person carries a defective or mutated form of a particular gene, that gene's protein product will not do the job it is intended to do. This can lead to disease. Most of our current therapies for such diseases are aimed at treating the symptoms of the diseases produced by the defective genes. Gene therapy is intended to cure the disease by replacing the defective gene with one that produces the correct protein. Genes can be attached to modified versions of viruses or similar structures that have the ability to penetrate the nucleus of a cell and become incorporated into the cell's existing DNA.

Gene therapy typically involves three "steps":

Administration: The introduction of the correct form of the gene into the body.
Delivery: The transfer of that correct gene to the nucleus of the cells for which it is intended.
Expression: The production of the proper protein by the cells that have received the corrected form of the gene.

The proper forms of the genes are either introduced in the laboratory or directly into the body. For laboratory procedures, the cells with the defective gene are removed from the body, the genes are introduced in culture dishes, and the altered cells returned to the body.

Alternatively, the corrected genes, attached to the vectors (often harmless viruses) that carry them into the nuclei, can be injected into the body. Current technology requires that such genes be injected quite near or directly into the tissue that needs them. Scientists are now investigating new techniques that would permit the injection to take place at any site.