The normal prion found in cattle shares two biochemical features with the prion found in humans and apes. The discovery is a shattering blow for the evolutionary argument that because humans seem unable to contract prion diseases from sheep infected with scrapie, they would be unlikely to contract CJD from contaminated beef.
Prions are proteins that occur in the brains of all mammals so far studied.
Sometimes, 'rogue' prions are produced by genetic mutations. This explains why some cases of CJD in humans are inherited. Another inherited human prion disease is so-called Fatal Familial Insomnia. There is some evidence that this results from a deficiency of the normal prion protein, a deficiency that occurs because mutant prions are unable to fulfil their normal functions. In addition to causing disease through inherited genetic mutations, mutant prions are capable of turning into 'rogue' disease agents.