11.   In gene therapy new genes are inserted into cells, often using a virus as the vector to deliver the genes.

12.   Instead, gene therapy researchers use viruses to carry in copies of entire normal genes in an effort to replace defective genes.

13.   It does not use viruses, and it does not attempt to replace an entire gene.

14.   Later, the related vaccinia virus was used for the purpose.

15.   Most experiments now use stripped-down viruses, but even with their infectious genes removed, these viruses can still stimulate potentially harmful immune responses.

16.   Most gene therapy experiments have used viruses to carry new genes into the body, a method that has sometimes caused inflammations and other immune reactions.

17.   Most approaches to gene therapy use viruses as the vehicle to convey genes to target cells.

18.   Other companies are using natural viruses to carry gene therapy.

19.   Onyx is also studying ways to use the virus to carry therapeutic genes to tumor cells.

20.   One way to reduce the risk, says Gary Nabel of the National Institutes of Health, is to use only viruses that cannot replicate.