The regulatory T cells (Tregs), formerly known as suppressor T cells, are a subpopulation of T cells which modulate the immune system, maintain tolerance to self-antigens, and abrogate autoimmune disease. These cells generally suppress or downregulate induction and proliferation of effector T cells. Additional regulatory T cells known as Treg17 cells have recently been identified. Mouse models have suggested that modulation of Tregs can treat autoimmune disease and cancer, and facilitate organ transplantation.