Some viruses show promise in terms of preferentially attacking cancer cells - but they can be engineered to be far more effective in the role of therapeutic agent: "Parvoviruses specifically kill cancer cells and are already in the clinical trial stage for treating malignant brain tumors. However, they can also infect normal cells - without doing any harm to them - so a large portion of viruses is lost during therapy. [Researchers] have now modified parvoviruses in such a way that they initially lose their ability to infect cells. In a second step, they equipped the viruses with a molecular key for infecting cancer cells. ... the researchers chose H1 parvoviruses, which normally infect rodents but are also infectious for human cells. H1 viruses kill tumor cells on the basis of their natural properties, i.e., their genetic material does not need to be modified for them to do so. ... The viruses exclusively destroy cancer cells. But with the same efficiency that they infect cancer cells, they also infect healthy cells. There they do not cause any damage and cannot replicate, but we lose a large portion of therapeutic viruses every time ... To solve this problem, the researchers [first] modified the genetic material of the virus in such a way that it loses its ability to infect cells. In a second step, this non-infectious virus was equipped with a molecular key for cancer cells. ... This is first evidence that it is basically possible to modify properties of H1 according to a plan. We will surely need several more attempts in order to target the viruses more specifically to cancer cells in the second step. We also already have ideas how to further enhance the infectious capacity and the potential to destroy cancer cells."