CJD comprises six major subtypes with highly heterogeneous clinico‐pathological phenotypes, which are largely determined and classified by the genotype at the polymorphic codon 129 (encoding methionine, M or valine, V) in PRNP, and by the type (type 1 or type 2) of abnormal prion protein (PrPSc) accumulating in the brain (e.g., MM1, VV1, MM2, VV2, MV2).5 The gene discussed is PRNP; the disease is Creutzfeldt Jacob disease.