4-1BBL–/– mice also generated functionally impaired CD8 T cells during infection with latent mouse gammaherpesvirus-68 (MHV-68), but in this case their numbers were unaltered (Fuse et al., 2007), and 4-1BBL–/– mice or wild-type mice injected with a neutralizing antibody to 4-1BBL generated lower late but not acute CD8 T cell responses to MCMV (Humphreys et al., 2010). The gene discussed is CD8A; the disease is infection.