The PC61.5 antibody reacts with mouse CD25 (low-affinity interleukin‐2 receptor α chain or IL-2Rα), which is an ~55 kDa type 1 transmembrane glycoprotein expressed on T and B cell progenitors, activated (but not resting) T and B cells, regulatory T (Treg) cells, dendritic cells, and activated monocytes and macrophages. Expression of CD25, together with CD4 and FOXP3, is considered a phenotypic signature for Treg cells. CD25 per se has low affinity for its IL-2 ligand but associates with CD122 (IL-2Rβ) and CD132 (IL-2Rγ) to form the high-affinity IL-2R receptor. CD25 is necessary for receptor clustering and induction of signaling pathways involved in the activation and proliferation of lymphocytes. Binding of the PC61.5 antibody inhibits binding of IL-2 to the low- and high-affinity receptors, which may result in inhibition of IL-2-dependent proliferation. The epitope recognized by PC61.5 is distinct from the IL-2 binding site and from the epitopes recognized by antibodies 7D4 and 3C7.