Oncogenes are genes that actively cause tumor growth.
Tumor suppressor genes actively stop tumor growth until they are altered or mutated.
Current estimates are that about 50 different types of cancers have genetic bases and that each of these may be associated with its own tumor suppressor gene. The VHL gene produces a protein that consists of 213 amino acids. VHL and VEGF, along with several other genes and their protein products, sense and respond to changes in the local tissue environment. An active VHL gene suppresses excessive VEGF activity, and this inhibits the formation of unnecessary blood vessels. This is how a tumor arises.