Religious proscriptions may be violated, and food preferences, such as vegetarianism, may become difficult to follow. In addition to genes altering key features of the target organism, other genes that produce novel proteins are usually transferred as well so that the genetic engineer can know that a gene has been successfully transferred. These novel proteins may cause allergic reactions in unsuspecting people. Most of these products are still in the experimental phase. Labeling is not now required on these products when they come to market.
Plant breeders can select genes which confer herbicide in new crop varieties.
Most herbicides are used before planting because they would harm the plant if applied later. With increased herbicide use comes a greater risk of groundwater contamination. The introduction of novel organisms can also affect the ecology of a field and its environs. For example, one microorganism engineered to better convert crop wastes to ethanol was found by its designers to also harm a beneficial fungi in the soil, reducing the capacity of nearby plants to absorb essential nitrogen. Molecular biologists are defining an increasing number of human diseases as genetic.