Human Foxp3 (Mr ~57 kDa). Does not cross react with Mouse Foxp3. T regulatory cells (Originally called T suppressor cells) are a subpopulation of T cells. Their major roles are to suppress excess immune response of other cells and to maintain tolerance of self-antigens. Initially, researchers used CD4 and CD25 (IL-2 receptor) to identify T suppress subpopulation and purified CD4+CD25+ T cells were used for T cells Suppression Assay, however some other activated T cells also express CD25.1,2 The most convincible molecular marker of Treg cells is Forkhead box protein 3 (Foxp3) which is expressed in the nucleus.3 Anti-Foxp3 antibodies are now widely applied to identify the roles of Treg cells in the development of many diseases, such as cancer, autoimmune disease, and organ transplantation.4,5,6 Scientists found that the over-activation of Treg cells may cause cancer cells to escape from immune surveillance 5. Conversely, patients with autoimmune diseases such as psoriasis, systemic lupus erythematous (SLE) have relative dysfunction of Foxp3 positive cells.7,8 Immunologists are trying to induce immune tolerance in organ transplantation using Treg cells.6