PRNP and scrapie: We worked with a panel of strains that were biologically cloned on homogeneous genetic backgrounds, obtained after transmission of either classical and atypical (Nor98) sheep scrapie and BSE, or hamster scrapie infectious sources in transgenic mice expressing ovine PrP (VRQ allele; tg338 mice) and hamster PrP (tg7 line), respectively.