Two unusual phenotypes in Saccharomyces cerevisiae, [PSI+] and [URE3], have been suggested to be due to prion proteins. Various research groups have shown that this is indeed the case and have characterized these yeast prions in more detail. The factors involved in prion formation, such as chaperone protein, and the intramolecular determinants of prion formation have been investigated. The ability of these yeast proteins to form prion is due to modular domains conserved throughout evolution.