Our study reveals a lower prevalence of the CD8+ T cell response to EBV in inactive MS patients, a higher frequency of CD8+ T cells specific for EBV lytic and latent antigens during active and inactive disease, respectively, and marked changes in the EBV-specific CD8+ T cell response during treatment with approved disease-modifying drugs, such as interferon-β (IFN-β) and natalizumab. The gene discussed is IFNB1; the disease is myeloid sarcoma.