In 1965, Brian Cox, a geneticist working with the yeast "Saccharomyces cerevisiae", described a genetic trait (termed with an unusual pattern of inheritance. The initial discovery of [PSI+ was made in a strain auxotrophic for adenine due to a nonsense mutation. Despite many years of effort, Cox could not identify a conventional mutation that was responsible for the trait. In 1994, yeast geneticist Reed Wickner correctly hypothesized that [PSI+ as well as another mysterious heritable trait, , resulted from prion forms of the normal cellular proteins, Sup35p and Ure2p, respectively. The names of yeast prions are frequently placed within brackets to indicate that they are non-mendelian in their passage to progeny cells, much like plasmid and mitochondrial DNA.