PRNP and scrapie: In a previous study [24], the shortest incubation periods following experimental infection of sheep with scrapie were recorded when the PRNP genotype of the source inoculum was matched to that of the sheep being challenged (e.g. when VRQ/VRQ sheep were inoculated with brain homogenate from VRQ/VRQ, rather than ARQ/ARQ, scrapie cases), and similar results have been reported by others [25].