cancer. Merkel cell polyomavirusa polyoma virusis associated with the development of Merkel cell carcinoma RNA viruses Not all oncoviruses are DNA viruses. Some RNA viruses have also been associated such as the hepatitis C virus as well as certain retroviruses, e.g., human T-lymphotropic virus (HTLV-1) and Rous sarcoma virus (RSV). Overview table Estimated percent of new cancers attributable to the virus worldwide in 2002. NA indicates not available. The association of other viruses with human cancer is continually under research. Main viruses associated with human cancer The main viruses associated with human cancers are the human papillomavirus, the hepatitis B and hepatitis C viruses, the Epstein–Barr virus, the human T-lymphotropic virus, the Kaposi's sarcoma-associated herpesvirus (KSHV) and the Merkel cell polyomavirus. Experimental and epidemiological data imply a causative role for viruses and they appear to be the second most important risk factor for cancer development in humans, exceeded only by tobacco usage. The mode of virally induced tumors can be divided into two, acutely transforming or slowly transforming. In acutely transforming viruses, the viral particles carry a gene that encodes for an overactive oncogene called viral-oncogene (v-onc), and the infected cell is transformed as soon as v-onc is expressed. In contrast, in slowly transforming viruses, the virus genome is inserted, especially as viral genome insertion is an obligatory part of retroviruses, near a proto-oncogene in the host genome. The viral promoter or other transcription regulation elements in turn cause overexpression of that proto-oncogene, which in turn induces uncontrolled cellular proliferation. Because viral genome insertion is not specific to proto-oncogenes and the chance of insertion near that proto-oncogene is low, slowly transforming viruses have very long tumor latency compared to acutely transforming viruses, which already carry the viral oncogene. Hepatitis viruses, including hepatitis B and hepatitis C, can induce a chronic viral infection that leads to liver cancer in 0.47% of hepatitis B patients per year (especially in Asia, less so in North America), and in 1.4% of hepatitis C carriers per year. Liver cirrhosis, whether from chronic viral hepatitis infection or alcoholism, is associated with the development of liver cancer, and the combination of cirrhosis and viral hepatitis presents the highest risk of liver cancer development. Worldwide, liver cancer is one of the most common, and most deadly, cancers due to a huge burden of viral hepatitis transmission and disease. Through advances in cancer research, vaccines designed to prevent cancer