Early in the course of MS an abrupt increase inthe EBV-infected blast population during intercurrent infections might be able tostimulate proliferation of EBV-latent-specific CD4+ T cells andCD8+ T cells, but with increasing duration of MS the capacity ofthese T cells to respond diminishes, as shown for the EBV-specificCD4+ T-cell population in Figure 2d,the diminution occurring as a result of T-cell exhaustion and possibly other factors,for example an age-related decline in the tendency of EBV to reactivate. The gene discussed is CD4; the disease is myeloid sarcoma.