Traditionally, the inherited cases have been classified as Gerstmann–Sträussler–Scheinker disease (GSS), fatal familial insomnia (FFI) or CJD; however, the range of clinical presentations, even within families with the same PRNP mutation [18–22], has led to sub-classification based on the PRNP mutation and the genotype at codon 129 [20,23]. Here, PRNP is linked to fatal familial insomnia.