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different mechanisms are involved in viral oncogenesis:
i)DNA viruses produce early proteins and gene promoters which bind to DNA and stimulate uncontolled cell division without virus particle production. eg bovine papilloma virus and Marek's disease.
ii)Leukaemia viruses turn on host cellular proto-oncogenes which are adjacent to where their proviral DNA is integrated (termed insertional mutagenesis). This is a rare event and may never happen in the lifetime of the animal.
(iii)The sarcoma viruses have an oncogene in their own genome. They cause transformation in-vitro and rapid fibrosarcomas in-vivo and are very rare.