Our findings, showing that IL-2 increases IL-10–producing memory CD4+ T cells, may also provide an explanation for the defective IL-10 production, but normal Foxp3 gene expression, in CD4+ T cells from a patient with CD25 deficiency, manifesting as immune dysregulation, polyendocrinopathy, enteropathy, and X-linked–like syndrome (54). Here, CD4 is linked to hyperinsulinemic hypoglycemia, familial, 4.