PRNP and human prion disease: PrP can adopt at least two conformations: a "cellular" isoform (PrPc) anchored at the membrane by a glycosylphosphatidylinositol (GPI) residue and constitutively expressed on many tissues, and a "scrapie" isoform (PrPSc) rich in beta-pleated sheets, resistant to managed protease treatments and viewed as the propagating agent of human and animal transmissible spongiform encephalopathies (TSE) or prion diseases [2].