[URE3] is a non-Mendelian genetic element that mimics recessive mutations in the chromosomal URE2 gene making cells derepressed for nitrogen catabolic enzymes. [PSI] is a non-Mendelian enhancer of readthrough of translational termination similar in its effects to some mutations in the chromosomal SUP35 gene. Three lines of evidence led to the proposal75 that both [URE3] and [PSI] are prions, infectious proteins analogous to the scrapie agent mediating transmissible spongiform encephalopathies of mammals. (1) Both [PSI] and [URE3] are reversibly curable. (2) [PSI] propagation requires SUP35 and [URE3] propagation requires URE2 with recessive chromosomal mutants having the same phenotypes as the presence of the respective dominant non-Mendelian element. (3) Overproduction of Sup35p and Ure2p increases the frequency of cells acquiring [PSI] or [URE3], respectively.