PRNP and prion disease: Most significantly, CJD, by far the most prevalent prion disease in humans, comprises six subtypes, which are primarily determined by the genotype at the polymorphic codon 129 (encoding methionine, M or valine, V) in PRNP, and by the type (1 or 2) of abnormal prion protein (PrPSc) accumulating in the brain (i.e., MM1, VV1, MM2, VV2, MV2) [63–65] (Table 1).