 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.Only a single person with vCJD tested was found to be heterozygous; most of those affected had two copies of the methionine-containing form.It is not yet known whether those unaffected are actually immune or only have a longer incubation period until symptoms appear.