A possible genetically engineered substance deemed unfit for human consumption has been found in taco shells bearing the Taco Bell brand, according to The Washington Post. The shells, distributed by Kraft Foods, are found on grocery store shelves across America. Shells sold at the chain’s restaurants have not been tested yet, but will be soon.

Testing methods for the shells are tricky, but, according to an independent lab, accurate tests have been duplicated multiple times, leaving little doubt of the presence of bioengineered proteins that may cause allergic reactions in humans.

The FDA has been asked to recall the shells, which now appear to have been made from disease-resistant corn.

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