The regulatory T cells (Tregs or Treg cells), formerly known as suppressor T cells, are a subpopulation of T cells that modulate the immune system, maintain tolerance to self-antigens, and prevent autoimmune disease. Treg cells are immunosuppressive and generally suppress or downregulate induction and proliferation of effector T cells. Treg cells express the biomarkers CD4, FOXP3, and CD25 and are thought to be derived from the same lineage as naïve CD4+ cells. Because effector T cells also express CD4 and CD25, Treg cells are very difficult to effectively discern from effector CD4+, making them difficult to study. Research has found that the cytokine transforming growth factor beta (TGF-β) is essential for Treg cells to differentiate from naïve CD4+ cells and is important in maintaining Treg cell homeostasis. Mouse models have suggested that modulation of Treg cells can treat autoimmune disease and cancer and can facilitate organ transplantation and wound healing. Their implications for cancer are complicated. Treg cells tend to be upregulated in individuals with cancer, and they seem to be recruited to the site of many tumors. Studies in both humans and animal models have implicated that high numbers of Treg cells in the tumor microenvironment is indicative of a poor prognosis, and Treg cells are thought to suppress tumor immunity, thus hindering the body's innate ability to control the growth of cancerous cells. Immunotherapy research is studying how regulation of T cells could possibly be utilized in the treatment of cancer. Populations T regulatory cells are a component of the immune system that suppress immune responses of other cells. This is an important "self-check" built into the immune system to prevent excessive reactions. Regulatory T cells come in many forms with the most well-understood being those that express CD4, CD25, and FOXP3 (CD4+CD25+ regulatory T cells). These Treg cells are different from helper T cells. Another regulatory T cell subset is Treg17 cells. Regulatory T cells are involved in shutting down immune responses after they have successfully eliminated invading organisms, and also in preventing autoimmunity. CD4+ FOXP3+ CD25(high) regulatory T cells have been called "naturally occurring" regulatory T cells to distinguish them from "suppressor" T cell populations that are generated in vitro. Additional regulatory T cell populations include Tr1, Th3, CD8+CD28−, and Qa-1 restricted T cells. The contribution of these populations to self-tolerance and immune homeostasis is less well defined. FOXP3 can be used as a good marker