Transmissable spongiform encephalopathy (TSE) agents as crystalline forms of the prion protein (PrP) that multiply by allowing normal metabolic forms of PrP to join the crystal.

The prion protein (PrP), is found only in the brain of animals infected with TSE. It is a modified form of a normal protein (PrPn) produced from the genome of the animal. The modification prevents breakdown by proteinases and hence its total chemical or physical structure is unknown. The finding of fibre-like structures in microglia and neurones that cross-react with antibodies produced against PrP and the rapid turnover of PrPn may mean that normal biochemical pathway PrPn forms can join a crystal seed of PrP to produce these fibres. This hypothesis, that the modification of PrP is physical rather than chemical, avoids the major problems with theories of PrP as the infective agent.