The vCJD prion strain (type 4 PrPSc propagated on human PrP 129M) is clearly the dominant prion strain in the 129MV vCJD patient’s brain and at primary transmission the majority of recipient humanized transgenic mice showed phenotypes that were entirely consistent with those from our historical transmission series of numerous 129MM vCJD patient brain samples. This evidence concerns the gene PRNP and variant Creutzfeldt-Jakob disease.