PRNP and Creutzfeldt Jacob disease: Human PrP has a common polymorphism at residue 129 (encoding either methionine or valine) and homozygosity confers genetic susceptibility to both sporadic and acquired forms of CJD (Collinge et al. 1991; Palmer et al. 1991; Windl et al. 1996; Lee et al. 2001; Mead et al. 2003; Collins et al. 2006).