Prions are proteinaceous infectious particles that lack nucleic acids. Prions are composed largely, if not entirely, of an abnormal isoform of a normal cellular protein.
In mammals, prions are composed of an abnormal, pathogenic isoform of the prion protein (PrP). A chromosomal gene encodes PrP and no PrP genes were found in purified preparations of prions. Neither prion specific nucleic acids nor virus-like particles have been detected in purified, infectious preparations.
In fungi, evidence for three different prions has been accumulated. Those studies are summarized in the section on Fungal Prions.
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