The latter two genes (SH2D1A and BIRC4) encode slam associated protein (SAP) and X-linked inhibitor of apoptosis protein (XIAP) and defects are identified in boys with X-linked lymphoproliferative disease (XLP types 1 and 2), a primary immunodeficiency that may present with HLH, especially upon infection with Epstein Barr Virus (EBV) (Marsh and Filipovich, 2011). Here, XIAP is linked to hemophagocytic syndrome.