HLH turned out to be characteristic of patients with X-linked lymphoproliferative syndrome (XIAP and SH2D1A patients) and was also found in ten patients with TNFRSF9 (51), LRBA (67), ITK (53, 68), RASGRP1 (69) deficiencies, CTLA4 haploinsufficiency (55, 70), CARD11 GOF (71) and PIK3CD GOF (52) (Table 1). The gene discussed is XIAP; the disease is X-linked lymphoproliferative syndrome.