PRNP and scrapie: It has now been established that susceptibility to scrapie in sheep is predominantly influenced by PRNP polymorphisms at codons 136 (alanine [A136] or valine [V136]), 154 (arginine [R154] or histidine [H154]) and 171 (glutamine [Q171] or R171), with ARQ/ARR and ARR/ARR being highly resistant against classical scrapie and VRQ/VRQ, ARQ/ARQ and VRQ/ARQ being highly susceptible [7].