Classical XLP due to SAP deficiency (XLP1) is characterized by susceptibility to fulminant Epstein-Barr virus (EBV) infection, frequently leading to haemophagocytic lymphohistiocytosis (HLH), development of lymphoma and hypogammaglobulinemia (Purtilo et al, 1975). Here, SH2D1A is linked to X-linked lymphoproliferative disease.