CD25/IL-2 receptor alpha is a transmembrane protein that associates with IL-2 R beta/CD122 and the Common gamma Chain/CD132 on activated T cells and regulatory T cells (Treg). The Common gamma chain is a shared subunit in the receptors for IL-4, -7, -9, -15, and -21. IL-2 R alpha is required for mediating IL-2 induced effects including T cell proliferation, activation induced cell death (AICD) of autoreactive naive T cells, and the development of CD4+CD25+ Treg which contribute to peripheral T cell homeostasis. A soluble form of IL-2 R alpha can be generated by proteolytic cleavage of the cell surface receptor.