PRNP and prion disease: Given the high susceptibility of bank voles to many prion diseases [52] and the development of a sporadic prion disease in transgenic mice overexpressing this protein bearing an isoleucine polymorphism at position 109 [18], we investigated whether the presence of isoleucine at this position is critical for such behavior and if spontaneous misfolding could occur using recombinant PrP from other species.