However, not everyone is convinced that genetically modified foods are beneficial. Opponents say that too little research has been done on the effects of GMOs on humans and on the environment. They argue that GMOs could be creating new poisons. They are also concerned about what might happen if genetically modified plants breed with wild plants. Some farmers are concerned about the possibility of inadvertently crossbreeding GMO plants with non-GMO plants. For example, if the pollen from a GMO plant blows into their organic field, now that organic crop could become a GMO crop. The possibility also exists that all this interbreeding could lead to new strains of herbicide-resistant super-weeds. What is the topic sentence of this paragraph?