The prion hypothesis states that the partially protease-resistant and detergent-insoluble prion protein (PrP(Sc)) is identical with the infectious agent, and lacks any detectable nucleic acids. Since the latter discovery, transgenic mice have contributed many important insights to the field of prion biology. The prion protein (PrP(C)) is encoded by the Prnp gene, and disruption of Prnp leads to resistance to infection by prions. Ectopic expression of PrP(C) in PrP(C)-knockout mice proved a useful tool for the identification of host cells competent for prion replication. Finally, the availability of PrP(C)-knockout mice, and transgenic mice overexpressing PrP(C), allowed selective reconstitution experiments aimed at expressing PrP(C) in neurografts or in specific populations of hemato- and lymphopoietic cells. The latter studies helped elucidate some of the mechanisms of prion spread and disease pathogenesis.