In many cases, such transformed cells were shown to induce tumors upon transplantation into animals. The most important step in cell transformation is immortalization , an ability to divide indefinitely. Immortalized cells have to undergo further genetic mutation before acquiring the full capacity to grow into a tumor. Animal viruses that oncogenic include Marek s disease virus, avian leukemia virus and papillomaviruses. The mechanism of virus-induced cellular transformation is poorly understood.
Oncogenes are genes whose products cause transformation or tumorigenesis.
Cells also encode some genes whose functions to stop cell growth and division called tumor suppressor genes .