PRNP and infection: In vivo, the need to distinguish the three types of PrP molecules involved in establishing prion infection (exogenous PrPSc, host cell PrPC, and host cell-derived PrPSc), as well as the rapid dissemination of prions out of the brain following intracranial inoculation [5] has made it difficult to detect exogenous PrPSc within the first few days post-infection [6, 7].