Washington ? Scientists said Sunday that they have used genetic engineering techniques to produce the first cattle that may be biologically incapable of getting mad cow disease.

The animals, which lack a gene that is crucial to the disease’s progression, were not designed for use as food. They were created so that human pharmaceuticals can be made in their blood without the danger that those products might get contaminated with the infectious agent that causes mad cow.

That agent, a protein known as a prion (pronounced PREE-on), can cause a fatal human ailment, variant Creutzfeldt-Jakob disease, if it gets into the body.

More generally, scientists said, the animals will facilitate studies of prions, which are among the strangest of all known infectious agents because they do not contain any genetic material. Prions also cause scrapie in sheep and fatal wasting diseases in elk and minks.

In the future, experts said, similar techniques might be used to engineer animals with more nutritious meats – though the Food and Drug Administration has said it will require engineered food animals to pass tests far more stringent than those it recently deemed adequate for clones.

“This is a seminal research paper,” said Barbara Glenn, director for animal biotechnology at the Biotechnology Industry Organization, a Washington industry group that counts among its members Hematech, the Sioux Falls, S.D.-based company that created the gene-altered cattle.

“This shows the application of transgenics to improving livestock production and ultimately food production.”

Prions, which are normal protein components of the brain, immune system and other tissues, cause disease only when they “go bad.” For these long strands of protein, that means folding themselves into three-dimensional shapes that are slightly different from their conventional conformation.

Prions remain poorly understood, but experiments suggest that it takes just one bad one to ruin a brain. That’s because a badly folded prion in the brain can strong-arm normal, nearby prions, turning good prions bad.

So, although prions are not able to replicate themselves the way bacteria and viruses do – by creating new offspring – they can amplify their numbers and spread disease as long as there are normal prions around to be recruited.