MAS, also known as secondary hemophagocytic lymphohistiocytosis (HLH) is a systemic hyper-inflammatory state resulting from abnormal natural killer and T-cell activation leading to a cytokine storm and ultimately the over-activation of macrophages producing an excessive inflammatory response and hyper-secretion of cytokines such as interferon gamma, tumor necrosis factor, interleukin (IL)-1, IL-6, IL-10, IL-12, IL-18, and macrophage colony-stimulating factor. The gene discussed is TNF; the disease is macrophage activation syndrome.