Host genetic factors (KLRC2 genotype for NKG2C+ NK cells and NKG2D genotype for NKG2D+ NK cells affecting NK cell activation and proliferation) and virus-related factors (Human Cytomegalovirus [HCMV] infection and HCMV UL40 variants affecting NKG2C+ NK cell response) could mediate NK cell function and thus influencing the risk of MS. The gene discussed is KLRK1; the disease is myeloid sarcoma.