[PSI+] turns 50

The year 2015 sees the fiftieth anniversary of the publication of a research paper that underpins much of our understanding of fungal prion biology, namely “?, a cytoplasmic suppressor of super-suppressor in yeast” by Brian Cox. Here we show how our understanding of the molecular nature of the [PSI+] determinant evolved from an ‘occult’ determinant to a transmissible amyloid form of a translation termination factor. We also consider the impact studies on [PSI] have had – and continue to have - on prion research. To demonstrate this, leading investigators in the yeast prion field who have made extensive use of the [PSI+] trait in their research, provide their own commentaries on the discovery and significance of [PSI].