Because human PrP with 129 valine appears to be incompatible with the PrPSc conformation propagated in vCJD [49] (Figure 2), and may have a dominant negative influence on the propagation of the vCJD prion strain in codon 129 heterozygous mice [171,172], this could explain why all neuropathologically confirmed cases of vCJD have been in individuals homozygous for 129 methionine. Here, PRNP is linked to variant Creutzfeldt-Jakob disease.