Scientists have genetically engineered a dozen cows to be free from the proteins that cause mad cow disease, a breakthrough that may make the animals immune to the brain-wasting disease.

An international team of researchers from the U.S. and Japan reported Sunday that they had "knocked out" the gene responsible for making the proteins, called prions. The disease didn't take hold when brain tissue from two of the genetically engineered cows was exposed to bad prions in the laboratory, they said.

Experts said the work may offer another layer of security to people concerned about eating infected beef, although though any food derived from genetically engineered animals must first be approved by the Food and Drug Administration.

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