Human epithelialA type of tissue (epithelium) that covers our exposed surfaces, such as skin. Also lines our hollow or tube-like organs/tissues such as the digestive tract. Since these tissues are often exposed to environmental insults such as chemicals and solar radiation and are often divide rapidly to replace lost cells, many cancers arise in epithelial tissues. and fibroblast cells are transformed into tumor cells for the first time in a laboratory. This was accomplished by the coexpression of telomeraseAn enzyme that functions to replace the ends of chromosomes. Normally, during DNA replication, chromosome ends are shortened by a small amount. Telomerase is turned off in most adult tissues, a process that limits the number of cell divisions that can be completed by those cells. In cancer cells, telomerase is often reactivated, allowing the cells to divide indefinitely. (hTERT), the simian virusThe smallest type of organism known. A single virus particle (called a virion) is much smaller than a cell. Viruses reproduce by invading cells and forcing the cells to make progeny virions. Viruses generally have specific host cells which they infect. Some virus types are associated with specific cancers and can convert normal cells into cancer cells. Examples of viruses associated with cancer are: papillomavirus-cervical cancer, hepatitis virus-liver cancer, Epstein-Barr virus-Burkitt's lymphoma. Virus-induced transformation of cells was one of the very first model systems for the study of cancer. Viruses have also been a key tool in the identification of many oncogenes. Because of their great ability to enter target cells viruses such as adenoviruses are being examined for their ability to deliver genes to cancer cells. 40 large-T oncoprotein, and an oncogenic alleleDifferent versions of the same gene are called alleles. Humans have two copies of most genes because one copy is inherited from each parent. of H-RASA proto-oncogene that is found to be mutated in many different kinds of cancer. The ras protein is involved in transmitting signals through the cell that drive the cell into the division process..1
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