The HLH-2004 protocol provides a framework for diagnosing HLH, outlining clinical, laboratory, and pathological criteria, including fever, splenomegaly, cytopenia, hypertriglyceridemia, hypofibrinogenemia, hemophagocytosis in hematopoietic or lymphoid organs, low or absent NK cell activity, hyperferritinemia, elevated soluble CD25, and elevated chemokine (C-X-C motif) ligand 9 (CXCL9) [12]. The gene discussed is CXCL9; the disease is isolated hyperferritinemia.