These include the p.W688* mutation in CIITA in patient 3, previously reported in a patient with a compound heterozygous MHC Class II deficiency (28), and the p.R1445Q substitution in LRBA in patient 2, which led to loss of CTLA4 and immune dysregulation in a patient when biallelic (20). Here, CTLA4 is linked to hyperinsulinemic hypoglycemia, familial, 4.