Prions are among the most mysterious creatures ever produced in nature. Lacking nucleic acid genomes and composed entirely of proteins, these infectious agents are not eliminated by any traditional sterilization procedures. Prions cause mad cow disease and related disorders in mammals including humans (1). It also appears that proteins with prion properties are widespread in nature and can be found in organisms that are very distant from mammals, for example, in yeast (2, 3). Although yeast prions are likely to be harmful to their hosts (3), they do not kill yeast cells outright and can be propagated through an indefinite number of generations. In this issue of PNAS, Chang et al. (4) use a yeast prion model for deciphering the protein regions that determine prion identities.
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