Oncogenes are genes that control the rate of production of proteins that regulate cell growth. When an oncogene stops functioning normally, cells begin producing too much oncogene protein, which, in turn, causes the cells to begin reproducing too rapidly. The resultant uncontrolled cell proliferation (cell reproduction) can lead to cancer. Cancers that result from the expression of too many oncogene proteins (such as HER-2/neu) on the surface of the cells are frequently more aggressive and more likely to recur than are other cancers.
Oncogene overexpression means that too many copies of the oncogene are present in the nuclei of the cells. If several copies of the oncogene are active (“turned on”) in medical lingo, too much oncogene protein is produced, leading to uncontrolled cell growth.
An example of the process is that overexpression of the HER-2/neu gene in breast cells leads to abnormally high numbers of HER-2 receptors on the surface of the cells, which results in binding of many estrogen molecules that can stimulate the breast cell to continue to multiply. Eventually this process can lead to breast cancer. Many estrogen receptor-positive breast cancer cells exhibit abnormally high numbers of HER-2 receptors on the surface.