Several studies have established that polymorphisms at sheep PrP amino acid position 136, 154 and 171 surrounding [140-PLIHFGNDY-148] are most relevant in differential TSE susceptibility [5,59-63] and that stability of this PrP region is a crucial determinant in whether PrPC is converted [40,64] (affecting species-barrier and/or scrapie susceptibility). This evidence concerns the gene PRNP and scrapie.