A murine perforin-deficient model of HLH has been generated which accurately recapitulates the immunological characteristics of the disease.5 Correction of immune dysregulation in this model by transplantation with wild-type (WT) bone marrow has been demonstrated, and in these studies, the prevention of HLH development is critically dependent upon the engraftment of functional CD8+ T cells.6 This evidence concerns the gene PRF1 and hemophagocytic syndrome.