PRNP and prion disease: Prion diseases are under strong genetic control, with the most powerful risk factors being located at the PrP gene locus (PRNP), particularly the polymorphism at codon 129 in the PRNP gene where methionine (∼ 60% allele frequency in Europeans) or valine is encoded and both homozygous genotypes are at increased risk of disease [11, 45].