To exclude the possibility that the phenotypes we observed in MHC Class I×β2M-/- and MHC Class II-/- mice were due to abnormal immune ontogeny in knockout mice, we determined the requirement for CD4 and CD8 T cells in the clearance of primary MNV infection in WT mice depleted of CD4 and CD8 T cells. The gene discussed is CD8A; the disease is infection.