Hemophagocytic lymphocytosis/macrophage activation syndrome (HLH/MAS) is a group of acute systemic inflammatory response syndromes caused by abnormal activation of cytotoxic lymphocytes and macrophages, which secrete many pro-inflammatory cytokines such as interferon γ (IFN-γ), tumor necrosis factor α (TNF-α), and interleukin (IL)-1, IL-4, IL-6, IL-8, IL-10, and IL-18. The gene discussed is IL18; the disease is macrophage activation syndrome.