Abstract
A group of brain diseases, including BSE (or mad cow disease) and Creutzfeldt-Jacob disease of humans, are transmitted by "prions." In his Perspective, Cheseboro outlines the evidence that prions are composed only of protein (an idea advanced by recent Nobel Prize winner Stanley Prusiner) and the evidence against that hypothesis. He cautions against complacency regarding the obstacles remaining in the field, since, he believes, the central core of the problem remains unresolved.