PRNP and prion disease: Polymorphism at residue 129 of human PrP [where either methionine (M) or valine (V) can be encoded] not only affects susceptibility to sporadic and acquired human prion diseases (Palmer et al., 1991; Collinge et al., 1991; Lee et al., 2001; Mead et al., 2003), but can affect the age of onset and also modify the phenotypes of IPDs (Baker et al., 1991; Poulter et al., 1992; Goldfarb et al., 1992; Mead et al., 2006).