The storm of scientific criticism over claims that a genetically modified (GM) maize causes severe disease in rats shows no signs of abating. Gilles-Eric Séralini, a molecular biologist at the University of Caen, France, is under intense pressure to report the full data behind his team’s finding that rats fed for two years with Monsanto’s glyphosate-resistant NK603 maize (corn) developed many more tumours and died earlier than controls (see Nature 489, 484; 2012).

Europe has never been particularly fond of genetically modified (GM) foods, but a startling research paper looks set to harden public and political opposition even further, despite a torrent of scepticism from scientists about the work. The study, published in the peer-reviewed journal Food and Chemical Toxicology, looked for adverse health effects in rats fed NK603 maize (corn), developed by biotech company Monsanto to resist the herbicide glyphosate and approved for animal and human consumption
in the European Union, United States and other countries.

The health effects of a Roundup-tolerant genetically modified maize (from 11% in the diet), cultivated with or without Roundup, and Roundup alone (from 0.1 ppb in water), were studied 2 years in rats. In females, all treated groups died 2–3 times more than controls, and more rapidly. This difference was visible in 3 male groups fed GMOs. All results were hormone and sex dependent, and the pathological profiles were comparable. Females developed large mammary tumors almost always more often than and