PRNP and human prion disease: The accumulation in the central nervous system of a post-translationally altered isoform (PrPSc) of the cellular prion protein (PrPC) is the key event in TSE pathogenesis [1], Nevertheless the relationships between PrPSc and infectivity are not definitively clear and evidences for high titers of TSE infectivity associated with extremely low levels of PrPSc have been reported [9].