According to hematologist Arthur Nienhuis, former president of the American Society of Gene Therapy, gene therapy:
....is the addition of new genes to a patient’s cells to replace missing or malfunctioning genes. Researchers typically use a virus to carry the genetic cargo into cells, because that is what viruses evolved to do with their own genetic material.
In 2008, Arthur Nienhuis reported that gene therapy cured 90% of the 30 treated patients with a genetic disorder called severe combined immunodeficiency disease (SCID), children who cannot ward off routine infections and die in childhood (bubble-boys). The gene therapists inserted the therapeutic gene in the SCID patients using a category of virus known as retroviruses, which have an easy time inserting their genes among the DNA of the patient.  The method has the possibility the retrovirus will insert its genes where it can adversely affect other genes and cause leukemia. Fortunately the risk seems small, and the leukemia mostly curable. With larger numbers of patients treated and followed throughout their life, gene therapist can assess the full potential for cure and for adverse effects in SCID.
Nienhuis also pointed out that gene therapists have targeted a wide range of medical conditions for gene therapy application:
Cancer, genetic disorders, immunologic disorders such as graft -vs.-host disease, neurodegenerative disorders, and infectious diseases, including AIDS, have all become targets for gene therapy applications.
Some target specifically diseases resulting from genetically defective bone marrow cells, which provide the for red blood cells and other elements of blood and the cells that maintain bone as a healthy organ. They do bench gene therapy by inserting genes in immature bone marrow cells harvested from the patient, then replace the genetically modified immature bone marrow cells so that they can ultimately replace all the defective cells in the bone.
Researchers have devised other methods, not using viruses as vectors, to insert normal genes into the human genome, methods at various stages of experimental development. The Human Genome Project Information website, maintained by the Office of Science of the U.S. Department of Energy, describes those other methods. That website also contains a summary of the fundamentals of the various aspects of gene therapy, including a description of the basic approaches to gene therapy, the current status of gene therapy research and clinical application, updates on recent developments, and a discussion of ethical considerations regarding gene therapy.
As of July, 2007, The Federal Drug Administration (FDA) had not approved any human gene therapy product for sale. 
However, the amount of gene-related research and development occurring in the United States continues to grow at a fast rate and FDA is actively involved in overseeing this activity. FDA has received many requests from medical researchers and manufacturers to study gene therapy and to develop gene therapy products. Such research could lead to gene-based treatments for cancer, cystic fibrosis, heart disease, hemophilia, wounds, infectious diseases such as AIDS, and graft-versus-host disease.
As of July, 2008, FDA had received its first application for approval of a gene therapy product, for treatment of selected patients with head and neck cancer.
- Nienhuis A. (2008) How does gene therapy work? Scientific American. August, page 108.
- • Arthur Nienhaus , former president of the American Society of Gene Therapy, holds a position as hematologist at St. Judes Children’s Research Hospital, Memphis, Tenn
- Nienhuis A. (2008) The Growing Clinical Impact of Gene Therapy. Mol Ther 16:995-996
- Dimauro S, Hirano M, Schon EA. (2006) Mitochondrial Medicine. Abingdon, Oxon: Informa Healthcare. ISBN 978-1-84214-288-2.
- • Table of contents: Introduction : the birth of mitochondrial medicine; The mitochondrial respiratory chain and its disorders; Mitochondrial neurology I : encephalopathies; Mitochondrial neurology II : myopathies and peripheral neuropathies; Mitochondrial cardiology; Mitochondrial ophthalmology; Mitochondrial gastroenterology; Mitochondrial otology; Mitochondrial endocrinology; Mitochondrial nephrology; Mitochondrial hematology and oncology; Mitochondrial reproductive medicine; Mitochondrial psychiatry; Mitochondrial dysfunction and neurodegenerative disorders; Therapeutic approaches.
- Kyriakouli DS, Boesch P, Taylor RW, Lightowlers RN. (2008) Progress and prospects: gene therapy for mitochondrial DNA disease. Gene Ther 15: 1017-1023; advance online publication, May 22, 2008. (Full-Test Free)
- Human Genome Project Information: Gene Therapy
- Excellent introduction to fundamentals of gene therapy. Site includes sections entitled:
- What is gene therapy?
- How does gene therapy work?
- What is the current status of gene therapy research?
- What factors have kept gene therapy from becoming an effective treatment for genetic disease?
- What are some recent developments in gene therapy research?
- What are some of the ethical considerations for using gene therapy?
- Cellular and Gene Therapy, Center for Biologics Information and Research, U.S. Food and Drug Information
- Introgen Therapeutics' application