CD25 (IL2 receptor alpha chain/IL2RA) is a cytokine that plays a role in the proliferation of T and B lymphocytes. The receptor of this cytokine (IL2RA) is a heterotrimeric protein complex with a gamma chain also shared by interleukin 4 (IL4) and interleukin 7 (IL7). IL2RA, IL2R beta chain (IL2RB), and the IL2R gamma chain (IL2RG), constitute the high-affinity IL2 receptor. Homodimeric IL2RA chains result in low-affinity receptor, while homodimeric IL2RB chains produce a medium-affinity receptor. The expression of IL2 in mature thymocytes is monoallelic, which represents an unusual regulatory mode for controlling the precise expression of a single gene. IL2 is primarily produced by mature T cells. IL2 plays an important role as a growth factor, differentiation factor, and regulator of cell death. IL-2 stimulates the proliferation of B cells, augments natural killer cell activity, and inhibits granulocyte macrophage colony formation. The targeted disruption of a similar gene in mice leads to ulcerative colitis-like disease, which suggests a role in the immune response to antigenic stimuli. Mutations in this gene are associated with interleukin 2 receptor alpha deficiency.