According to the "protein-only" hypothesis, the prion protein seemed to be - for more than a decade - the infectious agent of transmissible spongiform encephalopathies (TSEs) in different species. TSEs are neurodegenerative diseases reported e.g., for humans (Creutzfeld-Jakob disease, CJD), cattle (bovine spongiform encephalopathy, BSE), and sheep (scrapie). In contrast to the "protein-only" hypothesis, some researchers published data showing that retrovirus-like RNA transcripts, a viral structure, and viral particles might be involved in disease infection/progression of CJD. In addition, there is no final proof for the "protein-only" hypothesis and the infectivity of the prion protein. more