Specific pathogen-free B6 and BALB/c mice were obtained from National Cancer Institute (Bethesda, MD). To obtain infected mice in which CTL escape at S510 was detected, suckling B6 mice were infected intranasally with 2-4610 4 PFU of recombinant JHMV at 10 days of age and nursed by dams that were immunized with JHMV, as described previously [27] . For experiments comparing multiple JHMV variants, each litter served as an internal control: equal numbers of pups were infected with rJ and one to three recombinant variant viruses, depending on litter size. All animal studies were approved by the University of Iowa Animal Care and Use Committee.