ICOS and hyperinsulinemic hypoglycemia, familial, 4: The following gene defects were identified: DOCK8 (n = 10), DOCK2 (n = 3), TFRC deficiency (n = 8), ICOS deficiency (n = 2), RFXANK resulting in MHC class II deficiency (n = 12 mutations), ZAP-70 (n = 1), IKBKB (n = 1).