X-linked lymphoproliferative syndrome (XLP) is a rare primary immunodeficiency disease that can be divided into two types: SAP deficiency (XLP1) and XIAP deficiency (XLP2), caused by mutations in the SH2D1A and XIAP genes, respectively. The gene discussed is SH2D1A; the disease is hyperinsulinemic hypoglycemia, familial, 4.