Perhaps the most promising way to reach tumor cells is through viruses. In gene therapy, weakened viruses can act as couriers that deliver normal genes into cells. The best of these viruses in terms of its potential ability to deliver therapeutic genes to cancer cells is the adenovirus. If a gene useful for therapy is spliced into the viral DNA, the virus will deliver the needed gene into any cell it invades. The virus will do no harm as long as its own genes that confer virulence are removed when the new gene is inserted.
Adenoviruses can also kill tumor cells specifically.
When a virus enters a normal cell, so-called p53 proteins respond by instructing the infected cell to stop making DNA, thus preventing the virus from replicating. An adenovirus protein can bind directly to p53 and thereby disable it. Then the virus can use the cell's machinery to replicate itself. The adenovirus can be genetically altered in such a way that it assumes command of tumor cells only, not healthy ones. Specifically, the p53-binding protein of the adenovirus can be made so that it can no longer bind to p53.