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An oncogene is a gene that causes a cell to develop into a tumor cell.

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Protooncogene 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:
  • A mutation within a protooncogene can cause a change in the protein structure, caused by
  • An increase in protein concentration, caused by
    • an increase of protein expression (through misregulation)
    • an increase of protein stability, prolonging its existence and thus its activity in the cell
    • a gene duplication[?], resulting in a doubled amount of protein in the cell


Growth factors

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.

Protein kinases

There are six known classes of tyrosine kinases that can become an oncogene:
  1. Receptor tyrosine kinases that become constitutive (permanently) active.
  2. Cytoplasmic tyrosine kinases, often products of viral oncogenes.
  3. Regulatory GTPases, for example, the Ras protein[?].
  4. Cytoplasmic Serine/Threonine kinases and their regulatory subunits, for example, the Raf kinase[?], and cyclines[?] (through overexpression).
  5. Adaptor proteins[?] in signal transduction.
  6. Transcription factors.

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