Primary HLH is caused by mutations in genes implicated in granule-mediated cytotoxicity, impairing the function of natural killer (NK) and CD8+ cytotoxic T lymphocytes (CTLs) (3), or can develop as a complication in X-linked lymphoproliferative disease (XLP; XLP1 and XLP2) in which mutations in SAP or XIAP confer an increased susceptibility to HLH, particularly following infection with Epstein–Barr virus (EBV) (4, 5). This evidence concerns the gene XIAP and X-linked lymphoproliferative disease.