The current classification of sporadic Creutzfeldt–Jakob disease (sCJD), the most common human prion disease, recognizes six major phenotypic subtypes with distinctive clinicopathological features, broadly correlating at the molecular level with the genotype at the polymorphic codon 129 (methionine, M; or valine, V) in the prion protein gene (PRNP) and the size of the protease-resistant PrPSc core (type 1 migrating at 21 kDa and type 2 at 19 kDa), which is a prion strain marker in individuals with the same codon 129 genotype [79, 92]. This evidence concerns the gene PRNP and prion disease.