PRNP and scrapie: In contrast, disease-associated isoforms of PrP that comprise infectious prion assemblies are found only in prion-infected tissue and are composed of detergent-insoluble polymeric PrP structures some of which become protease-resistant and are classically termed PrPSc (the scrapie isoform; Meyer et al., 1986; Prusiner, 1987, 1991, 1998).