PRNP and Creutzfeldt Jacob disease: A molecular classification scheme for CJD that considers the apparent molecular weight of the PK-resistant unglycosylated fragment of PrPSc (21 or 19 kDa) and the methionine (M)/valine (V) polymorphism at codon-129 (MM, VV or MV) on the prion protein gene (PRNP) has been adopted worldwide4–6.