The diagnosis needs either a molecular diagnosis consistent with HLH including the identification of pathologic mutations of PRF1, UNC13D, or STX11 or the presence of at least five out of eight features: fever, splenomegaly, bi/pancytopenia, hypertriglyceridemia or hypofibrinogenemia, ferritin >500 ng/ml, hemophagocytosis in the bone marrow/lymph nodes/spleen, low natural killer cell activity, soluble CD25 (soluble interleukin-2 receptor) >2400 U/ml [7, 8]. The gene discussed is PRF1; the disease is hemophagocytic syndrome.