Efficiency of PMCA conversion also depended on a human PrP polymorphism at position 129 [methionine (M)/valine (V)], affecting susceptibility to prion disease in humans [5, 18, 42], especially in the only to date known human prion disease (vCJD) acquired after animal prions (BSE) crossed the species barrier [14]. This evidence concerns the gene PRNP and prion disease.