PrPSc was defined on 3F4-stained immunohistochemistry slides of brain tissue as fine, coarse or plaque-like deposits of brown-stained material which was seen in tg66 mice injected with brain from familial prion disease patients with PrP mutations, but which was not seen in aged-matched uninjected control tg66 mice. Furthermore, by these criteria, PrPSc was not detected by immunohistochemistry (IHC) in tg66 mice injected with Q227X human brain (see results). Therefore, Q227X-injected mice also served as age-matched negative controls.