CD25 is a 55 kD glycoprotein, also known as the low affinity IL-2Rα, Ly-43, p55, or Tac. It is expressed on activated T and B cells, thymocyte subset, pre-B cells, and T regulatory cells. In association with CD122 (IL-2Rβ) and CD132(common γ chain), CD25 forms the high affinity signaling IL-2 receptor.