Numerous viruses are used in the service of science today. They serve as gene taxis to transfer therapeutic genes into body cells or as therapeutic viruses targeted to infect and destroy cancer cells. For such applications, the viruses are often equipped with additional genes, such as for immune mediators or for proteins inducing programmed cell death. However, these gene products can harm the body if they are released at the wrong moment or at excessive levels. "Ideally, we want to be able to turn on and off the transferred genes at a specific time," says Dr. Dirk Nettelbeck, a virologist from DKFZ.