Yeast prions (infectious proteins) were discovered by their outré genetic 
properties and have become important models for an array of human prion and 
amyloid diseases. A single prion protein can become any of many distinct amyloid 
forms (called prion variants or strains), each of which is self-propagating, but 
with different biological properties (eg, lethal vs mild). The folded 
in-register parallel β sheet architecture of the yeast prion amyloids naturally 
suggests a mechanism by which prion variant information can be faithfully 
transmitted for many generations. The yeast prions rely on cellular chaperones 
for their propagation, but can be cured by various chaperone imbalances. The 
Btn2/Cur1 system normally cures most variants of the [URE3] prion that arise. 
Although most variants of the [PSI+] and [URE3] prions are toxic or lethal, some 
are mild in their effects. Even the most mild forms of these prions are rare in 
the wild, indicating that they too are detrimental to yeast. The beneficial 
[Het-s] prion of Podospora anserina poses an important contrast in its 
structure, biology, and evolution to the yeast prions characterized thus far.