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

Table of contents
1 Protooncogene
2 Oncogene


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

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.