PRNP and infection: The essential role of host PrP for prion propagation and pathogenesis is demonstrated by the fact that knockout mice lacking PrP expression (Prnpo/o mice) are entirely resistant to prion infection19,20 and that reintroduction of PrP transgenes restores susceptibility to infection in a species-specific manner that allows reverse genetics approaches to studying structure–function relationships in PrP (for reviews see10,11,21–25).