Table 1.

Concepts linking yeast prions and subtypes of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD).

Amyloid	A structurally ordered filamentous aggregate consisting of a single protein in a polymeric arrangement; each end of the filament serves as template for the addition of more protein, providing a molecular mechanism for runaway aggregation. Several ALS-linked proteins can form amyloid with atypical dye-binding properties.
Neuronal cytoplasmic inclusions (NCIs)	Large pathological protein aggregates that commonly form in neurons afflicted by various neurodegenerative diseases. Different diseases frequently have a unique subset of proteins within NCIs. Similar inclusions are observed in yeast cells harboring specific prion proteins.
Prion	An infectious protein. Canonically, a prion protein adopts an alternative structural fold that causes similar protein molecules to adopt the same structure (e.g., amyloid). Most characterized prion proteins are found in fungi, particularly Saccharomyces cerevisiae.
Prion domain	The distinct segments of yeast proteins that enable prion formation. These domains are distinguished by their “low-complexity,” consisting predominately of hydrophilic, but not charged or hydrophobic, amino acids.
Prion-like domain	Segments found in many mammalian proteins, especially proteins with RNA-related functions, which have sequence composition very similar to yeast prion domains. They are disproportionately present in proteins that aggregate in neurodegenerative diseases.