PRF1 and hemophagocytic syndrome: To understand the possible mechanisms of HMB transformation into fulminant HLH, we evaluated and compared immunologic changes of immunoglobulins, lymphocyte subsets, cytotoxicity, intracellular perforin and granzyme expressions, EBV virus load and known candidate genes in patients with the episodes of HMB-CAEBV and EBV-HLH.