SH2D1A and X-linked lymphoproliferative disease: 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).