Not exactly. A 2012 French study found that rats fed a lifetime diet of genetically modified corn that made it resistant to the herbicide Roundup— or else given water containing Roundup — suffered tumors and organ damage. The researchers reported that 50 percent of male rats and 70 percent of females died prematurely, compared with only 30 percent of males and 20 percent of females in a control group [source: Reuters].
That made for a scary-sounding headline. But as routinely happens in the world of science, other researchers who had come to different conclusions in their own work started scrutinizing the study. Six French scientific academies quickly issued a rebuttal in which they found fault with the design of the experiment, the statistical analysis and the amount of data. The European Food Safety Authority declared that the study was "of insufficient scientific quality to be considered as valid for risk assessment." Additionally, the strain of rat that the researchers picked tends to get breast tumors easily, and can get them from overeating or eating corn contaminated by a common fungus that causes a hormone imbalance. The study didn't screen for those factors, so it's unclear whether the rats' tumors were caused by the GMO corn [source: MacKenzie].