Describing a chemical, organism, or environmental factor that causes the development of cancer. Some viruses are oncogenic to vertebrates, notably the retroviruses (including the Rous sarcoma virus of chickens), avian sarcoma virus, mouse mammary tumour virus, simian virus 40, human papillomavirus, and human adenovirus. These viruses contain genes (known as oncogenes) that become integrated into the host cell’s DNA and are responsible for the transformation of a normal cell into a cancerous cell. See also growth factor.