This includes CJD, which has been caused accidentally by medical procedures, as well as Fatal Familial Insomnia (FFI) and Gerstmann-Straussler-Scheinker Syndrome (GSS), rare forms of human transmissible spongiform encephalopathy known to be caused by genetic mutations of the prion protein.

Scientists explained this result by saying that blood cells in these mice don't contain any normal PrP and therefore can't carry the infection to the brain, says Pierluigi Gambetti at Case Western Reserve University in Cleveland, who studies an inherited prion disease called fatal familial insomnia, which makes people unable to sleep and eventually causes death.

For example, people with fatal familial insomnia and those with inherited Creutzfeldt-Jakob disease have an altered amino acid in PrP at position 178.