PRNP and prion disease: Human prion diseases are biologically unique in that the disease process can be triggered through inherited germ line mutations in the human prion protein gene (PRNP), infection (by inoculation, or in some cases by dietary exposure) with prion-infected tissue or by rare sporadic events that generate PrPSc (Prusiner 1998; Collinge 2001, 2005; Weissmann 2004; Wadsworth & Collinge 2007).