Rous’s sarcoma virus, then, was the product of an incredible evolutionary accident. Retroviruses, Temin had shown, shuttle constantly out of the cell’s genome: RNA to DNA to RNA. During this cycling, they can pick up pieces of the cell’s genes and carry them, like barnacles, from one cell to another. Rous’s sarcoma virus had likely picked up an activated src gene from a cancer cell and carried it in the viral genome, creating more cancer. The virus, in effect, was no more than an accidental courier for a gene that had originated in a cancer cell—a parasite parasitized by cancer. Rous had been
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