Hence, the phenotypic effect of the D178N mutation depends on polymorphism at codon 129 of PRNP. Fatal familial insomnia (FFI) seems to be associated with a D178N mutation and methionine at codon 129 of PRNP, whereas the phenotype of sCJD was observed for the D178N mutation with valine at codon 129 [9]. Here, PRNP is linked to fatal familial insomnia.