In the human prion diseases, the presence of a polymorphic variant (methionine/methionine, methionine/valine or valine/valine) at codon 129 of the prion gene (PRNP) can determine disease susceptibility and disease phenotype in patients with prion disease35 through the production of distinct conformers of misfolded PrP (‘strains’). Here, PRNP is linked to prion disease.