Whereas, transgenic mice expressing human PrP 129 valine are highly susceptible to classical CJD prions regardless of the PrPSc type or codon 129 genotype of the inoculum (Collinge et al., 1995, 1996; Telling et al., 1995; Hill et al., 1997; Korth et al., 2003; Kobayashi et al., 2007; Wadsworth et al., 2008b), the absence of a transmission barrier to classical CJD prions is not uniformly observed in mice expressing human PrP 129 methionine in the absence of mouse PrP. Here, PRNP is linked to Creutzfeldt Jacob disease.