PRNP and variant Creutzfeldt-Jakob disease: Intriguingly, until 2017 [90], all probable/definite vCJD-reported cases were homozygous for methionine (MM) at codon 129 of the PRNP gene, thus, suggesting an absolute barrier displayed by a PrPC sequence associated with MV or VV genotype (i.e., no clinically affected subjects were MV or VV carriers).