In the mid-1960s, two unusual genetic determinants were discovered in the yeast Saccharomyces cerevisiae. In genetic crosses, these determinants, called [PSI+] and [URE3] respectively, were found not to be inherited according to Mendel's laws. Little did the researchers realize at the time that the non-Mendelian elements they had stumbled upon were in fact two different yeast prions – although definitive evidence of this only emerged some 30 years later1.

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