1. Colorado State University, Prion Research Center, Department of Microbiology, Immunology, and Pathology, College of Veterinary Medicine and Biomedical Sciences, Fort Collins, Colorado, USA
2. National Park Service, Biological Resources Division, 1201 Oakridge Drive, Suite 200, Fort Collins, Colorado, USA
3. Roslin Institute, University of Edinburgh, Easter Bush, Midlothian EH25 9RG, UK