PRNP and sporadic Creutzfeldt-Jakob disease: These two PrPSc isoforms, named PrPSc types 1 and 2, respectively, along with the methionine/valine polymorphism at codon 129 of the PrP gene, led to the current classification of sporadic CJD (sCJD) in 5 phenotypically distinct subtypes (2,3).