The host susceptibility is influenced by the prion protein-encoding gene PRNP. For example, the variant of Creutzfeldt-Jakob disease (vCJD), which has been associated to the classical bovine spongiform encephalopathy (C-BSE) epidemics in cattle through contaminated meat product consumption [2], has occurred so far only in individuals homozygous for methionine at codon 129 of PRNP [3]. The gene discussed is PRNP; the disease is Creutzfeldt Jacob disease.