Early studies of human prions used primates [55–57]; however, following the demonstration in 1995 that the species barrier limiting transmission of human prions to wild-type mice can be obviated by expression of human PrP in the absence of endogenous mouse PrP [58,59] such ‘humanised’ transgenic mice have become key experimental models for studying human prion disease [43,60–65]. The gene discussed is PRNP; the disease is prion disease.