CREUTZFELDT-JACOB DISEASE “in humans virtually every vCJD patient has been homozygous for methionine at codon 129, while heterozygotes at the same codon appear most resistant to kuru (a prion disease of Papua New Guinean Highlanders transmitted during cannibalistic mortuary feasts). In sheep, three PRNP codons (136, 154 and 171) are largely responsible for determining scrapie susceptibility.”
http://mbe.oxfordjournals.org/content/20/1/111.full