Unlike in fHLH and X-linked lymphoproliferative disease type 1, the cytotoxic function of NK cells and cytotoxic T-cells is not impaired (42, 43), which implies that a mechanism other than cytotoxic dysfunction could be involved in recurrent HLH in XIAP deficiency. Here, XIAP is linked to hyperinsulinemic hypoglycemia, familial, 4.