Transmissible spongiform encephalopathies (TSEs), including kuru and Creutzfeldt–Jakob disease of humans, scrapie of sheep, and bovine spongiform encephalopathy (BSE) of cattle, were previously classified as slow virus diseases. However, TSEs are more correctly classified as prion diseases. Prions are misfolded proteins that are "infectious" because they can induce misfolding in other previously normal proteins; however, they do not contain DNA or RNA so they cannot be classified as viruses. Before scientists knew the cause of spongiform encephalopathies, they hypothesized that small virus particles, which they termed virions, were to blame. Once it was discovered that prions were the real cause of TSEs and that prions contained no nucleic acid, the term virions was discarded and these particles were renamed prions. A minority of researchers still believe, however, that prion diseases are caused by an as-yet unidentified slow virus, although there is little evidence to support this finding, as Ma and colleagues have created a recombinant prion protein.