They hope the cattle can be the source of herds that can provide dairy products, gelatin and other products free of the brain-destroying disease, also known as bovine spongiform encephalopathy or BSE.

Writing in the journal Nature Biotechnology, the researchers said their cattle were healthy at the age of 20 months, and sperm from the males made normal embryos that were used to impregnate cows, although it is not certain yet that they could breed normally.

The cattle lack the nervous system prions, a type of protein, that cause BSE and other related diseases such as scrapie in sheep and Creutzfeldt-Jakob disease, known as CJD, in humans, the researchers said.

"(Prion-protein-negative) cattle could be a preferred source of a wide variety of bovine-derived products that have been extensively used in biotechnology, such as milk, gelatin, collagen, serum and plasma," they wrote in their report.

Yoshimi Kuroiwa of Kirin Brewery Co. <2503.T> in Tokyo, Japan and colleagues made the cattle, known as knockouts because a specific gene has been "knocked" out of them, using a method they call gene targeting.

"By knocking out the prion protein gene and producing healthy calves, our team has successfully demonstrated that normal cellular prion protein is not necessary for the normal development and survival of cattle. The cows are now nearly 2 years old and are completely healthy," said James Robl of Hematech, a South Dakota subsidiary of Kirin.

"We anticipate that prion protein-free cows will be useful models to study prion disease processes in both animals and humans," Robl, an expert in cloning technology, said in a statement.

Misfolded prion proteins are blamed for BSE and other, similar brain diseases. It is known that certain genetic variations make animals more susceptible to the diseases.

BSE swept through British herds in the 1980s and people began developing an odd, early-onset form of CJD called variant CJD or vCJD a few years later. CJD normally affects one in a million people globally, usually the elderly, as it has a long incubation period.

There is no cure and it is always fatal.

As of November 2006, 200 vCJD patients were reported worldwide, including 164 patients in Britain, 21 in France, 4 in the Republic of Ireland, 3 in the United States, 2 in the Netherlands and 1 each in Canada, Italy, Japan, Portugal, Saudi Arabia and Spain.

The disease may have first started to infect cattle when they were fed improperly processed remains of sheep, possibly sheep infected with scrapie. Although people are not known to have ever caught scrapie from eating sheep, BSE can be transmitted to humans.

BSE occasionally occurs in cattle outside Britain although it is now rare.