Equal PFU (2-4610 4 ) of rJ and rJ.S Q600Y were combined and inoculated intranasally into 5-week-old B6 and BALB/c mice. Total RNA was harvested from the brains of mice 7 days p.i. and the relative representation of WT (wild type) versus variant template was determined by RT-PCR and sequencing. This assay can specifically detect a given species of template when that species comprises at least 20% of a heterogeneous pool [50] . Primers used were : forward, AACCCCTCGTCTTGGAATAGGAGG-TATGG; and reverse, CCTACGGATTGAACGCTATCATT-GACTAAC. PCR products were sequenced directly by the University of Iowa DNA Core.