Because serious food reactions can kill, people need to know when genetically modified products might cause allergic reactions. Supporters of labelling also point to the principle of the consumer's right to know what is in their food. Opponents point out that we don't know exactly what is in our food at the moment anyway. Many plants contain natural toxins to protect them against insect attack. These toxins are not good for us, and yet when we eat a parsnip we are not told the concentration of the potentially carcinogenic chemical that occurs naturally.
More serious worries stem from the use of marker genes .
These are genes that are inserted into the genetically modified organism along with the desired gene. The presence of marker genes, which are easy to spot, allows researchers to recognise organisms that contain the desired gene. The problem arises with those marker genes that give antibiotic resistance to the organism. Some people believe it is risky to allow genetically modified plants with marker genes for antibiotic resistance into the environment. For example, the British government objected to a proposal to import genetically modified corn from the United States into Europe because the plants contained a marker gene for resistance to the commonly used antibiotic, ampicillin.