IL2RA, the high-affinity IL-2 receptor, is most highly expressed on Tregs and is critical for their survival and function through the actions of the transcription factor STAT5B. IL2RA- and STAT5B-deficient patients can also exhibit a combined immunodeficiency with autoimmunity, atopic dermatitis, and elevated IgE (123, 124) in addition to non-hematopoietic effects on growth found in STAT5B deficiency. This evidence concerns the gene STAT5B and immune system disorder.