PRNP and scrapie: The susceptibility of sheep to scrapie and BSE is strongly influenced by different polymorphisms of the PRNP gene that encodes for prion protein (PrP), with polymorphisms at codons 136 (A or V), 141 (L or F), 154 (R or H) and 171 (R, Q or H) demonstrated to be of major importance (for recent review, see [16]).