Gene therapy is a treatment type that means changing a defective gene that causes a disease. This can mean replacing the gene that doesn't work with a functioning gene, or even 'knocking out' the gene all together.
There are two methods of delivering the new gene to the affected DNA
1. Liposome
2. Viral
In CF this isn't especially effective, but it has cured a rare immunodeficiency disease. Gene therapy affects somatic cells only, it is not a germ line therapy.
Links
Human Genome Project
Extension:
Guardian Special Report