gene therapy:

an approach to preventing and/or treating disease by replacing, removing or introducing genes or otherwise manipulating genetic material. Examples include adding a gene to a cell to produce a specific missing protein, using antisense molecules to prevent viral replication, and altering CD4 cells to make them resistant to HIV infection. Genes may be introduced by direct injection or using a harmless viral vector to deliver genes into cells. 

A promising, yet still experimental, process that involves altering the genetic composition of cells for the treatment or prevention of disease. In some cases the goal is to have the genetically modified cells repair a genetic anomaly that results in the prevention of disease progression (i.e., enzyme replacement therapy in certain genetic diseases). In other cases, the goal is that the genetically modified cells serve to protect the body against toxicities or disease progression (i.e., the insertion of a gene that makes bone marrow cells resistance to high dose chemotherapy or insertion of a gene that confers HIV resistance). In the human blood system, only stem cells reproduce themselves and recreate other types of blood cells, making umbilical cord blood, with its rich concentration of stem cells, extremely valuable for gene therapy.