The description of the patient with CTLA-4 deficiency caused by a missense mutation at the same position (c.356) as in our case as well as the result of bioinformatic prediction combined with the information concerning our patient’s phenotype correlating with the genotype strongly suggest the diagnosis of CTLA-4 deficiency in our case, irrespective of a lack of functional tests, which could not be performed at that time due to their limited availability, but also because of the severe clinical condition of our patient requiring HSCT (7). This evidence concerns the gene CTLA4 and hyperinsulinemic hypoglycemia, familial, 4.