 One explanation for this can be found in the genetics of people with the disease.The human PRNP protein which is subverted in prion disease can occur with either methionine or valine at amino acid 129, without any apparent difference in normal function.Of the overall Caucasian population, about 40% have two methionine-containing alleles, 10% have two valine-containing alleles, and the other 50% are heterozygous at this position.