Scientists said yesterday 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.
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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.