GITR (glucocorticoid-induced TNFR-related gene) is a member of the TNF receptor superfamily, also known as TNFRSF18 and AITR (in humans). It is expressed at low levels on resting T lymphocytes and at high levels on CD25+ CD4+ Tregs. The expression of GITR on T cells can be upregulated upon activation. Interaction of GITR with its ligand (GITRL) has been demonstrated to augment T cell activation, proliferation, cytokine production as well as MAPKs and NF-κB activation, and abrogate the inhibitory function of CD25+ CD4+ Tregs. In vivo activation of GITR causes development of autoimmune diseases and restores the suppressed immune response.