PRNP and scrapie: Inconsistent evidence associating prion infectivity with platelets may be explained by the variability in surface PrPC expression levels of various blood fractions of rodent species vs bovine, ovine and cervid species that could modify a cell subset’s ability to accumulate and/or replicate the prion agent; scrapie strain of the blood donors; route of inoculation; and/or processing of platelets.