Recent advances in understanding and manipulating genes -the biological units of heredity -have set the stage for scientists to alter patients' genetic material to fight or prevent disease. One major goal of gene therapy is to supply cells with healthy copies of missing or flawed genes. This approach is revolutionary: Instead of giving a patient a drug to treat or control the symptoms of a genetic disorder, physicians attempt to correct the basic problem by altering the genetic makeup of some of the patient's cells. 
Hundreds of major health problems are influenced by gene functions. In the future, gene therapy could be used to treat many of these conditions. Theoretically, it could also be used to alter germ cells (egg or sperm) in order to prevent a genetic defect from being transmitted to future generations . However, the possibility of germ-line gene therapy is beset by difficult ethical and social questions as well as technical obstacles. Gene therapy could also be used as a drug delivery system. To accomplish this, a gene that produces a useful product would be inserted into the DNA of the patient's cells. For example, during blood vessel surgery, a gene that makes an anticlotting factor could be inserted into the DNA of cells lining blood vessels to help prevent dangerous blood clots from forming. Many other conditions might also lend themselves to treatment using this general approach. As medicine operates increasingly on the molecular level, using gene therapy for drug delivery could save much effort and expense. It could shortcut the lengthy and complicated process of collecting large amounts of a gene's protein product, purifying the product, formulating it as a drug, and administering it to the patient. 
However, gene therapy is still extremely new and highly experimental. The number of approved trials is small, and relatively few patients have been treated to date. In some current experiments, cells from the blood or bone marrow are removed from the patient and grown in the laboratory under conditions that encourage them to multiply. Then the desired gene is inserted into the cells with the help of a disabled virus, and the successfully altered cells are selected out, encouraged to multiply, and returned to the patient's body. In other cases, liposomes (fatty particles) or disabled  viruses may be used to deliver the gene directly to cells within the patient's body. 

