An oncogene is a gene that causes a cell to develop into a tumor cell.
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A protooncogene is a gene that is involved in signal transduction and execution of mitogenic signals, usually through its protein product. Upon activation, it (or its product) becomes a tumor inducing agent, an oncogene.
The protooncogene can become an oncogene by a relatively small modification of its original function. There are two basic activation types:
Growth factors are usually secreted by few special cells to induce cell proliferation in other cells. If a cell that usually does not produce growth factors suddenly starts to do so (because it developed an oncogene), it will thereby induce its own uncontrolled proliferation (autocrine loop), as well as the proliferation of neighbouring cells.