PRNP and prion disease: In human PrP 129MV mice expressing a 2:3 ratio of human PrP 129M:129V, prions from 129MM vCJD patient brain produce high attack rates of infection (typically 100%) and faithful propagation of type 4 PrPSc, however mice do not develop clinical signs of prion disease and remain subclinically infected to advanced old age [9].