A useful analogy to consider when thinking about tumor suppressors and oncogenes is an automobile. The proto-oncogenes would be in control of the movement of a car (the gas pedal in the animation below). When everything is functioning properly, the car moves only when the gas pedal is pushed. In normal cells, both internal and external signals control the activity of the oncogenes. In the animation below, these signals would be represented by the the 'X' shaped growth factor and the foot in the video portion.
A defective oncogene would be analogous to a gas pedal that is stuck in the 'on' position. There is no longer a need for signals to activate these genes. The car would go forward regardless of whether the pedal was pushed or not!
What this means for cells is that they divide continuously even in the absence of any signals telling them to divide. We have two copies of each gene and for oncogenes, a single defective copy is enough to cause a cell to divide.
Numerous genes have been identified as proto-oncogenes. Many of these genes are responsible for providing the positive signals that lead to cell division. Some proto-oncogenes work to regulate cell death. As stated in the introduction to this section, the defective versions of these genes, known as oncogenes, can cause a cell to divide in an unregulated manner. This growth can occur in the absence of normal pro-growth signals such as those provided by growth factors. A key feature of oncogene activity is that a single altered copy leads to unregulated growth. This is in contrast with tumor suppressor genes which must BOTH be defective to lead to abnormal cell division.
The proto-oncogenes that have been identified so far have many different functions in the cell. Despite the differences in their normal roles, these genes all contribute to unregulated cell division if they are present in a mutant (oncogenic) form. The mutant proteins often retain some of their capabilities but are no longer sensitive to the controls that regulate the normal form of the protein. Selected oncogenes that have been associated with numerous cancer types are described in more detail on the pages that follow. To learn about a particular gene, choose from the list.