An oncogene is a modified or mutated gene that makes a tumor cell more malignant. While most potentially tumor-creating cells die instead of reproducing, a cell with one or more oncogene is more likely to grow into a tumor and possibly cause cancer.
Normal genes that have the potential to become oncogenes are called proto-oncogenes. When activated through a small modification of its original function, the proto-oncogene becomes a full oncogene.
The two basic types of activation are a mutation that changes the protein structure that makes it hyperactive, or that creates a hybrid protein; or an increase in protein concentration caused by duplication of the protein's source gene, misregulation, or increased stability.
The protein itself does not create a cancerous tumor. If, however, the cell in which the protein is found starts secreting growth factors when it is not supposed to, it starts multiplying uncontrollably, as well as inducing neighboring cells to proliferate. This is the process that creates a malignant tumor.
Much oncology research today focuses on ways to eliminate oncogenes or catch them in their earliest stages.
Web Resources On Oncogene
Book Resources On Oncogene
Oncogenes by Geoffrey M. Cooper
The Oncogene and Tumour Suppressor Gene Factsbook by Robin Hesketh