PRNP and human prion disease: In two separate challenges from individual infected animals, we have shown TSE disease pathology in 40–70 % of recipient knock-in human PrP transgenic mice homozygous for methionine at codon 129 (HuMM mice), whereas the same animals showed no signs of disease when inoculated with caBSE (Bishop et al., 2006; Plinston et al., 2011; Wilson et al., 2013).