Scientists also are trying to use viruses that never or rarely jump into genomes, yet hang around in the cell for the long haul. Scientists at Stanford, Children's Hospital of Philadelphia and Avigen Inc., an Alameda-based biotech company, have treated hemophiliac dogs with a blood-clotting gene spliced into what may be such a viral vector.
Even if genes are inserted into the genome, there are other ways to cut down on risk.
In some trials -- such as the French case -- a patient's cells are genetically altered in a test tube, and then added back to the body. Theoretically, scientists could screen these cells before returning them, in order to weed out ones with insertions in dangerous places.
Alternatively, a so-called suicide gene could be put in the virus along with the therapeutic gene. If the cell turns rogue -- becomes cancerous, for instance -- scientists can give the patient a drug that will activate the suicide gene, and the rogue cells will then be killed. Such a strategy has proved successful in one gene therapy trial.
But the ideal solution is to perfectly fix a genetic error without introducing any extra DNA, like the deft replacement of a spent battery in a car.