existing information. To date no such effects have been shown with commercialized transgenic crop plants. The work of Ewen and Pusztai (1999) hints of some possible interaction between a lectin expressed in potato and alterations in the potato caused by the genetic engineering process. According to the study, diets containing genetically engineered potatoes expressing the lectin, Galanthus nivalis agglutinin (GNA), showed some effects on different parts of the rat gastrointestinal tract. Those effects fell into two categories, ones caused by the GNA transgene itself and others caused by pleiotropic effects of expressing the transgene. However, analysis of the work of Ewen and Pusztai by the Royal Society (Royal Society 1999) and by Kuiper et al. (1999) indicates that the study lacked scientific rigor. For example, data concerning the biochemical composition of the potatoes used in the study show that the nontransgenic variety differed significantly from the transgenic variety. These differences could be attributable to natural variations in potato lines and are not necessarily due to the genetic modification (Kuiper et al. 1999).

It is important to ask whether any such threats have resulted from more conventional genetic modification of agricultural crop plants (conventional pest-protected plants) and, if so, whether they can serve as examples for assessing the risks of transgenic pest-protected plants. The questions outlined below can be asked, with equal validity, of transgenic pest-protected or conventional pest-protected plants. It should be noted, however, that conventional breeding usually selects for endpoints that are almost always controlled by several genes. It could be reasoned that such selection because of genetic linkage would be more likely to select simultaneously and inadvertently for an additional undesirable characteristic than would the introduction of a single gene or even a small number of genes with transgenic techniques (section 2.4.2). Both animal and plant breeding have yielded examples of inadvertent selection of undesirable characteristics.

Is the Transgene Expressed in the Edible Part of the Plant?

If so, what are the potential effects on humans, domestic animals, and other nontarget animals? Toxicity testing can be carried out on the gene product or the edible part of the plant, and testing to predict potential effects on humans is carried out on laboratory animals and extrapolated to humans. Toxicity testing of chemicals that are macronutrients, such as proteins, has unique problems. The maximal tolerated dose, as determined in short term tests, is usually very high and palatability problems unrelated to toxicity can interfere with tests. Given the high dose, it is difficult to feed enough of the plant material to the test subjects without making substantial dietary changes. Suitable controls are difficult to de-



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