PRNP and fatal familial insomnia: While patients with IPD have traditionally been classified by the clinicopathological syndromes of Gerstmann–Sträussler–Scheinker disease (GSS), Creutzfeldt–Jakob disease (CJD) or fatal familial insomnia (FFI), the advent of molecular genetic diagnosis led to the recognition of considerable phenotypic heterogeneity even within families with the same PRNP mutation (Collinge et al., 1989, 1990, 1992; Mallucci et al., 1999; Kovacs et al., 2002; Wadsworth et al., 2006) and subclassification of IPD by pathogenic mutation was proposed (Collinge et al., 1992; Collinge & Prusiner, 1992).