Varmus and Bishop had demonstrated that precursors of oncogenes—protooncogenes—existed in all normal cells. They had found activated versions of the src proto-oncogene in Rous sarcoma virus. They had suggested that mutations in such internal genes caused cancer—but a crucial piece of evidence was still missing. If Varmus and Bishop were right, then mutated versions of such proto-oncogenes must exist inside cancer cells. But thus far, although other scientists had isolated an assortment of oncogenes from retroviruses, no one had isolated an activated, mutated oncogene out of a cancer cell.
