We further performed intracellular staining after 4 hr peptide stimulation in vitro at day 6 and found that CD8 T cells from X31 primed mice produced similar amount of IFN-γ when stimulated with either CANP366 or X31NP366 peptide (Fig. 5B), but not after primary infection, confirming the observation using the ELISPOT methods. This evidence concerns the gene CD8A and infection.