More than 200 cases of variant Creutzfeldt-Jakob disease, the human form of mad cow, have occurred worldwide since the 1990s. No accepted treatment exists for the devastatingly fatal disease or any of the others caused by infection with the malformed, malignant protein particles called prions. Giovanna R. Mallucci and her co-workers at the Institute of Neurology in London have performed an experiment in mice that could lay the groundwork for an eventual cure. Researchers genetically engineered mice to produce the protein PrP for only the first nine weeks after birth. PrP misfolds in the presence of prions, which causes it to produce more prions.