Variant Creutzfeldt-Jakob disease (vCJD) appears to be caused by infection with the bovine spongiform encephalopathy (BSE) agent. To date, all patients with vCJD are homozygous for methionine at codon 129 of the PrP gene. To investigate the relationship between polymorphism at codon 129 and susceptibility to BSE or vCJD prions, we performed splenic follicular dendritic cell assay with humanized knock-in mice through peripheral infection. All humanized knock-in mice showed little or no susceptibility to BSE prions. Only the subset of humanized knock-in mice with codon 129 Met/Met genotype showed weak susceptibility by Western blotting. Surprisingly, we succeeded in the transmission of vCJD prions to humanized knock-in mice not only with codon 129 Met/Met but also with codon 129 Met/Val. Humanized knock-in mice with codon 129 Val/Val were not susceptible. The results suggest that human heterozygotes at codon 129 are also at risk for secondary infection with vCJD. (c) 2006 Elsevier Inc. All rights reserved.