Not all T cells are effector cells of the anti-tumor immune system. One of the 
subpopulations of CD4+ T cells that express CD25+ and the transcription factor 
FOXP3, known as Regulator T cells (TReg), plays an essential role in maintaining 
tolerance and immune homeostasis preventing autoimmune diseases, minimalize 
chronic inflammatory diseases by enlisting various immunoregulatory mechanisms. 
The balance between effector T cells (Teff) and regulator T cells is crucial in 
determining the outcome of an immune response. Regarding tumors, activation or 
expansion of TReg cells reduces anti-tumor immunity. TReg cells inhibit the 
activation of CD4+ and CD8+ T cells and suppress anti-tumor activity in the 
tumor microenvironment. In addition, TReg cells also promote tumor angiogenesis 
both directly and indirectly to ensure oxygen and nutrient transport to the 
tumor. There is accumulating evidence showing a positive result that removing or 
suppressing TReg cells increases anti-tumor immune response. However, depletion 
of TReg cells will cause autoimmunity. One strategy to improve or restore tumor 
immunity is targeted therapy on the dominant effector TReg cells in tumor 
tissue. Various molecules such as CTLA-4, CD4, CD25, GITR, PD-1, OX40, ICOS are 
in clinical trials to assess their role in attenuating TReg cells' function.