The disorganization of multicellular architecture and function that constitutes a cancer is, although abnormal, not wholly chaotic. Specific genes determine the cancerous behavior of affected cells. In cancer-associated retroviruses the specific genes are called "viral oncogenes," and they seem, because of similar deoxynucleotide sequences that are demonstrable in the chromosomal complement of normal cells, to have been acquired from progenitor cellular oncogenes (c-oncogenes). C-oncogenes have been preserved throughout evolution, since near-identical sequences are found in vertebrates from fish to man, and some are even found in fruit flies and worms. This suggests that c-oncogenes likely determine important unidentified functions in normal cells.
C-oncogenes in tumors may be activated from the protooncogene state by insertion of viral genes nearby, in the absence of a viral oncogene, or by a chemical carcinogen. The gene product in the cancer cell may be a normal molecule in excess or, because of substitution of a
Holland JF. Neoplastic Diseases. JAMA. 1984;252(16):2191–2194. doi:10.1001/jama.1984.03350160059018
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