Immune deficiencies associated HLH occur with significant frequency in X-linked lymphoproliferative syndrome type 1 (XLP1) [17], XLP type 2 (XLP2) [18], and IL-2–inducible T cell kinase deficiency-associated lymphoproliferation [19], which are characterized by mutations in SH2D1A, XIAP, and ITK genes, respectively. The gene discussed is XIAP; the disease is X-linked lymphoproliferative syndrome.