In some cases, new characteristics introduced intentionally may create toxicity. The Bt toxin as it appears in the bacteria that produce it naturally is considered relatively safe for humans. Humans have little experience with exposure to this form of the toxin. Furthermore, in the past humans have had no opportunity or reason to ingest any form of the Bt toxin in large quantities. And of course, a pesticide engineered into every cell of a food source cannot simply be washed off before a meal.
Toxicity can also result from characteristics introduced unintentionally.
For example, a plant that ordinarily produces high amounts of a toxin in its leaves and low amounts in its fruit could unexpectedly begin to concentrate the toxin in its fruit after addition of a new gene. Unpleasant surprises of this sort can result from our ignorance about exactly how a foreign gene has been incorporated into the engineered cell. These newly-inserted genes may sometimes end up in the middle of existing genetic instructions, and may disrupt those instructions. A foreign gene could, for example, be inserted in the middle of an existing gene that instructs a plant to shut off production of a toxin in its fruit. The foreign gene could disrupt the functioning of this existing gene, causing the plant to produce abnormal levels of the toxin in its fruit.