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infect a human. The development of pathogen-free colonies of pigs would also greatly reduce the likelihood of such an occurrence.

The real advantage to using pigs is that they are easily bred, mature quickly, and their organs are of a comparable size to that of humans. In addition, pigs are amenable to genetic engineering, whereby the genes that encode transplantation antigens that would be recognized by a human recipient could be removed so that the resulting organs would not be recognized as foreign in the human. In addition, pigs have now been cloned, so that once such an antigen-free animal has been constructed, we could have a continuous source of immunologically nonstimulating organs available for transplantation into human patients. see also Agricultural Biotechnology; Cloning Organisms; Embryonic Stem Cells; Immune System Genetics.

Richard D. Karp

Bibliography

Colen, B. D. "Organ Concert." Time Magazine (Fall 1996): 70-74.

Goldsby, R. A., T. J. Kindt, and B. A. Osborne. Kuby Immunology, 4th ed. New York: W. H. Freeman, 2000.

Lanza, R. P., D. K. Cooper, and W. L. Chick. "Xenotransplantation." Scientific American 277, no. 7 (1997): 54-59.

Miklos, A. G., and D. J. Mooney. "Growing New Organs." Scientific American 280, no. 4 (1999): 60-65.

Roitt, Ivan M., Jonathan Brostoff, and David K. Male. Immunology. St. Louis: Mosby, 2001.

Transposable Genetic Elements

Transposable genetic elements (TEs) are segments of DNA that can be integrated into new chromosomal (genomic) locations either through direct DNA transfer (transposons), or via an RNA intermediate (retrotransposons). Pseudonyms for TEs include mobile elements, jumping genes, genomic parasites, and selfish DNA. TEs are known to be responsible for several human genetic diseases and may play a role in evolution in many species.

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