In gene therapy, you insert a therapeutic gene into a “disabled” retrovirus (a vector). The vector is then used, e.g., to infect bone-marrow stem cells from a patient, and the cells are then re-injected into the patient. The hope is that they will multiply into (in this case) normal immune cells. This was used to treat 10 kids in France in 2000-2002. However, 2 of the 10 developed leukemia-like conditions.

The ability of the viruses to insert themselves into DNA was also why they were able to activate a cancer-promoting gene (an oncogene).