The forcible insertion of GM genes to add a new trait to a crop unavoidably affects expression of other unrelated genes, which may or may not affect nutritional quality and toxicity. Such changes cannot be predicted prior to consumption because researchers do not know what they are looking for. Animal feeding trials are one of the few ways to identify possible risks from unintended changes in the GMO.
Rats are often used in GMO feeding trials because like humans, they are mammals and have a similar digestive system and metabolism.
Why this strain of rat?
The strain of rat used in Séralini’s experiment was the Sprague-Dawley, a common choice for long-term toxicity and carcinogenicity studies performed by industry and independent scientists alike.[1-6]
This was also the same strain of rat used by Monsanto in its short 90-day study on the same GM maize that Séralini studied over the long term. Séralini used the same strain of rat in order that his experiment would be comparable with Monsanto’s.
While critics of Séralini’s study claim that the Sprague-Dawley is an unusually tumour-prone strain of rat, in fact it is about as prone to developing tumours as humans living in industrialized countries. For this reason, researchers view it as an excellent human-equivalent model for long-term toxicity and carcinogenicity tests.
 Meyer H and Hilbeck A (2013). Rat feeding studies with genetically modified maize – a comparative evaluation of applied methods and risk assessment standards. Environmental Sciences Europe 25(33). Online publication prior to print.
 Klimisch HJ et al (1997). Long-term inhalation toxicity of N-vinylpyrrolidone-2 vapours. Studies in rats. Food Chem Toxicol 35(10-11): 1041-1060.
 Soffritti M et al (2006). Results of a long-term carcinogenicity bioassay on Sprague-Dawley rats exposed to sodium arsenite administered in drinking water. Ann N Y Acad Sci 1076: 578-591.
 Rapporteur member state Germany (1998). Monograph on glyphosate. Vol 3-1 Glyphosat 05. German Federal Agency for Consumer Protection and Food Safety (BVL). 1998. http://earthopensource.org/files/pdfs/Roundup-and...
 Rapporteur member state Germany. Monograph on glyphosate. Vol 3-1 Glyphosat 04. German Federal Agency for Consumer Protection and Food Safety (BVL). 1998. http://earthopensource.org/files/pdfs/Roundup-and...
 National Toxicology Program (2012). Toxicology/Carcinogenicity. http://ntp.niehs.nih.gov/?objectid=72015DAF-BDB7-...
 Hammond B et al (2004). Results of a 13 week safety assurance study with rats fed grain from glyphosate tolerant corn. Food Chem Toxicol 42(6): 1003-1014.
 Soffritti M et al (2006). Cancer prevention: The lesson from the lab. In: Biasco G, Tanneberger S, eds. Cancer Medicine at the Dawn of the 21st Century: The view from Bologna. Bologna: Bononia University Press: 49–64.