Compared with 43 active Still’s disease cases, PRF1-mutated HLH patients showed significantly higher rates of cytopenias, hepatomegaly, and central nervous system involvement, together with substantially elevated ferritin levels (9,193 vs 800 ng/mL, p = 0.0023), whereas C-reactive protein levels were comparable. The gene discussed is PRF1; the disease is hemophagocytic syndrome.