D178N, a missense mutation on codon 178 of PRNP (D178N) with the substitute of asparagine for aspartic acid, has been associated with the clinicopathological phenotype of either CJD or fatal familial insomnia (FFI) depending on the polymorphic change of the prion protein (PrP) at position 129 (D178N-129M/M is related to FFI while D178N-129 V/V related to CJD) [5–9]. Here, PRNP is linked to fatal familial insomnia.