A prion is an infectious protein horizontally transmitting a disease or trait 
without a required nucleic acid. Yeast and fungal prions are nonchromosomal 
genes composed of protein, generally an altered form of a protein that catalyzes 
the same alteration of the protein. Yeast prions are thus transmitted both 
vertically (as genes composed of protein) and horizontally (as infectious 
proteins, or prions). Formation of amyloids (linear ordered β-sheet-rich protein 
aggregates with β-strands perpendicular to the long axis of the filament) 
underlies most yeast and fungal prions, and a single prion protein can have any 
of several distinct self-propagating amyloid forms with different biological 
properties (prion variants). Here we review the mechanism of faithful templating 
of protein conformation, the biological roles of these prions, and their 
interactions with cellular chaperones, the Btn2 and Cur1 aggregate-handling 
systems, and other cellular factors governing prion generation and propagation. 
Human amyloidoses include the PrP-based prion conditions and many other, more 
common amyloid-based diseases, several of which show prion-like features. Yeast 
prions increasingly are serving as models for the understanding and treatment of 
many mammalian amyloidoses. Patients with different clinical pictures of the 
same amyloidosis may be the equivalent of yeasts with different prion variants.