Normally, viruses infect cells, produce more viruses, and infect more cells, but they do not directly affect the genetic makeup, the DNA, of the cell. Influenza virus, for instance, infects lung cells and produces more influenza virus, but it does not leave a permanent fingerprint in our genes; when the virus goes away, our DNA is left untouched. But Rous’s virus behaved differently. Rous sarcoma virus, having infected the cells, had physically attached itself to the cell’s DNA and thereby altered the cell’s genetic makeup, its genome. “The virus, in some structural as well as functional
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