USGS National Wildlife Health Center 1 article published in JoVE Medicine Assessing Transmissible Spongiform Encephalopathy Species Barriers with an In Vitro Prion Protein Conversion Assay Christopher J. Johnson1, Christina M. Carlson2, Aaron R. Morawski3, Alyson Manthei4, Neil R. Cashman5 1USGS National Wildlife Health Center, 2Department of Soil Science, University of Wisconsin–Madison, 3Laboratory of Immunology, National Institute of Allergy and Infectious Diseases, National Institutes of Health, 4Merial Veterinary Scholars Program, School of Veterinary Medicine, University of Wisconsin–Madison, 5Department of Neurology, University of British Columbia Measuring the barrier to the interspecies transmission of prion diseases is challenging and typically involves animal challenges or biochemical assays. Here, we present an in vitro prion protein conversion assay with the ability to predict species barriers.