PRNP and scrapie: In order to analyze the response of endogenous host PrPC to prion infection, Tg3F4 mice (i.e. Tg(WT-E1+/+) (from reference [19]) expressing 3F4 epitope tagged mouse PrPC (PrP-3F4) were inoculated intracranially (IC) with normal brain homogenate (NBH) from uninfected RML mice or with brain homogenate from RML mice infected with the ME7 strain of mouse scrapie.