Approximately 12% of the CD8+ cells in the CNS of MHV-A59-infected animals are specific for this epitope (as measured by an intracellular gamma interferon [IFN-γ] assay). This level of S598 epitope-specific CD8+ T cells is the same even when the S510 epitope is expressed (25). Furthermore, infection of β2-microglobulin-deficient mice (β2 M−/−) with MHV-A59 results in a highly lethal infection (7), suggesting that this epitope is important in controlling infection. (Alternatively, there may be as yet undefined epitopes in nonstructural proteins, which have not yet been investigated for CD8+ T epitopes.)