FIG. 1.

Definition of a prion. “Prion” means an infectious protein. Many mechanisms can be imagined for such an entity (48), including a protein that induces its own gene’s transcription (A); a self-propagating covalent protein modification, such as acetylation, in which the modified form of the protein is much better at self-acetylation than the unacetylated form (B); and a self-propagating change in conformation, such as amyloid formation—the likely mechanism for the known prions (C).