Two infectious proteins (prions) of Saccharomyces cerevisiae have been identified by their unusual genetic properties: (1) reversible curability, (2) de novo induction of the infectious prion form by over-production of the protein, and (3) similar phenotype of the prion and mutation in the chromosomal gene encoding the protein. [URE3] is an altered infectious form of the Urea protein, a regulator of nitrogen catabolism, while [PSI] is a prion of the Sup35 protein, a subunit of the translation termination factor. The altered form of each is inactive in its normal function, but is able to convert the corresponding normal protein into the same altered inactive state. The N-terminal parts of Ure2p and Sup35p (the "prion domains") are responsible for prion formation and propagation and are rich in asparagine and glutamine residues. Ure2p and Sup35p are aggregated in vivo in [URE3]- and [PSI]-containing cells, respectively. The prion domains can form amyloid in vitro, suggesting that amyloid formation is the basis of these two prion diseases. Yeast prions can be cured by growth on millimolar concentrations of guanidine. An excess or deficiency of the chaperone Hsp104 cures the [PST] prion. Overexpression of fragments of Ure2p or certain fusion proteins leads to curing of [URE3].