Genetic defects predisposing to primary HLH are found in autosomal recessive familial HLH (FHL) 2-5, Griscelli syndrome type 2 (GS2), Chediak-Higashi syndrome (CHS), X-linked lymphoproliferative disease type 1 (XLP1), and X-linked inhibitor of apoptosis (XIAP) deficiency. The gene discussed is XIAP; the disease is hyperinsulinemic hypoglycemia, familial, 4.