Several molecular defects known to lead to primary T-cell deficiency were examined in patient 1 and found to be normal, adenosine deaminase and purine nucleoside phosphorylase deficiency (examined by measuring the respective enzyme in peripheral blood erythrocytes), a defect in chromosome breakage repair (examined following treatment of cultured cells with Diepoxybutan), NBS1-gene deletion, and 22q11 microdeletion were excluded. The gene discussed is PNP; the disease is hyperinsulinemic hypoglycemia, familial, 4.