are good models for studying factors like chaperones that affect protein aggregation. Also, the IPOD is the sub-cellular site to which amyloidogenic proteins are sequestered in yeast, and where prions like [PSI+] may undergo maturation. Thus, prions also serve as substrates to understand the intracellular processing of protein aggregates such as amyloid. Laboratories commonly identify [PSI+] by growth of a strain auxotrophic for adenine on media lacking adenine, similar to that used by Cox et al. These strains cannot synthesize adenine due to a nonsense mutation in one of the enzymes involved in the biosynthetic pathway. When the strain is grown on yeast-extract/dextrose/peptone media (YPD), the blocked pathway results in buildup of a red-colored intermediate compound, which is exported from the cell due to its toxicity. Hence, color is an alternative method of identifying [PSI+] -- [PSI+] strains are white or pinkish in color, and [psi-] strains are red. A third method of identifying [PSI+] is by the presence of Sup35 in the pelleted fraction of cellular lysate. When exposed to certain adverse conditions, in some genetic backgrounds [PSI+] cells actually fare better than their prion-free siblings; this finding suggests that the ability to adopt a [PSI+] prion form may result from positive evolutionary selection. It has been speculated that the ability to convert between prion-infected and prion-free forms acts as an evolutionary capacitor to enable yeast to quickly and reversibly adapt in variable environments. Nevertheless, Reed Wickner maintains that [URE3] and [PSI+] are diseases, although this claim has been challenged using theoretical population genetic models. [PIN+] / [RNQ+] The term [PIN+] was coined by Liebman and colleagues from Psi-INducibility, to describe a genetic requirement for the formation of the [PSI+] prion. They showed that [PIN+] was required for the induction of most variants of the [PSI+] prion. Later they identified [PIN+] as the prion form of the RNQ1 protein The more precise name [RNQ+] is now sometimes used because other factors or prions can also have a Psi-inducing phenotype. A non-prion function of Rnq1 has not been definitively characterized. Though reasons for this are poorly understood, it is suggested that [PIN+] aggregates may act as "seeds" for the polymerization of [PSI+] and other prions. The basis of the [PIN+] prion is an amyloid form of Rnq1 arranged in in-register parallel beta sheets, like the amyloid form of Sup35. Due to similar amyloid structures, the [PIN+] prion may facilitate the