PRNP and scrapie: As an example, valine (V) at codon 136 of the PRNP gene confers susceptibility to classical and resistance to atypical scrapie; conversely, arginine (R) at codon 171 confers resistance to classical and susceptibility to atypical scrapie, and histidine (H) at 154 is associated with susceptibility to atypical scrapie [21,22].