Although activation and proliferation of T cells in IPEX patients may be affected by immunosuppressive drug treatment, important characteristics of FOXP3 isoforms can be deduced: FOXP3fl increases upon TCR stimulation and is indispensable to induce sustainable Treg-related phenotype and functions, whereas FOXP3Δ2Δ7 counteracts the action of FOXP3fl under pro-inflammatory conditions. The gene discussed is FOXP3; the disease is immune dysregulation-polyendocrinopathy-enteropathy-X-linked syndrome.