Homozygosity at PRNP codon 129 predisposes to the development of sporadic and acquired CJD [80–85] and is most strikingly observed in vCJD where all neuropathologically confirmed cases studied so far have been homozygous for codon 129 methionine of PRNP[38,39,49,50]. This evidence concerns the gene PRNP and Creutzfeldt Jacob disease.