Homozygosity at PRNP codon 129 predisposes to iatrogenic (Collinge et al. 1991) and sporadic CJD (Palmer et al. 1991) and results in a younger age at onset in some inherited prion diseases (Dlouhy et al. 1992; Poulter et al. 1992; Mead et al. 2006, 2007). This evidence concerns the gene PRNP and prion disease.