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 any food derived from genetically engineered animals must first be approved by the Food and Drug Administration.

"This research is a huge step forward for the use of animal biotechnology that benefits consumers," said Barbara Glenn of the Biotechnology Industry Organization, a Washington industry group that includes the company that sponsored the research as a member.

"This a plus for consumers worldwide."

The surviving cows are now being injected directly with mad cow disease, known as bovine spongiform encephalopathy, or BSE, to make certain the cattle are immune to it.

Those key results won't be known until later this year, at the earliest, according to the Sioux Falls, S.D. based biotechnology company Hematech Inc. that sponsored the research. It can take as long as two years for mad cow disease to be detected in infected animals.

The research published in the online journal Nature Biotechnology could be used as a tool that would help researchers better understand similar brain-wasting diseases in humans, Glenn and others said.

Scientists are still mystified by the biological purposes of normal prions, which humans also produce. But they believe that even one prion going bad can set off the always fatal and painful brain disease — known as variant Creutzfeldt-Jakob disease in humans.

Similar prion-based diseases also are found in sheep, deer and elk.