During infection with VACV-Lister we observed 4-1BB to be expressed on a large subset of B8R-reactive CD8 T cells at day 5 but it was down-regulated by day 7 (Figure 3A), in contrast to our prior observation with VACV-WR where ~30% of B8R-reactive T cells still expressed 4-1BB on day 7 (Zhao and Croft, 2012). This evidence concerns the gene CD8A and infection.