A number of chemical and physical agents are known to trigger the uncontrolled proliferation of cells that characterise cancers, but in the last two decades it has become clear that at least six types of human cancer can be virally induced. How do cells lose control of their division, and how are viruses able to bring this about? It is now known that cells contain genes called protooncogenes, involved in normal cell replication. They are normally under the control of other, tumour-suppressor genes, but these can be blocked by proteins encoded by certain DNA viruses. When this happens, the protooncogene functions as an oncogene, and cell division is allowed to proceed uncontrolled. Retro-viruses have a different mechanism; they carry their own, altered, version of the cellular oncogene, which becomes integrated into the host's genome and leads to uncontrolled cell growth. Retro-virus oncogenes are thought to have been acquired originally from human (or animal) genomes, with the RNA transcript becoming incorporated into the retrovirus particle.
An oncogene is a gene associated with the conversion of a cell to a cancerous form.
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