Glucocorticoid-induced TNFR-related protein (GITR), also known as CD357 or TNFRSF18, is a member of the tumor necrosis factor receptor (TNF-R) superfamily. GITR is expressed constitutively at high levels in T regulatory cells (Treg cells) and plays a key role in dominant immunological self-tolerance maintained by CD25+CD4+ regulatory T cells (PMID: 11812990). It is expressed at low levels on resting responder T cells. The expression of GITR on T cells can be upregulated upon activation (PMID: 15770698). GITR is activated by GITR ligand (GITRL) which is mainly expressed on APC. GITR-GITRL interactions could co-stimulate both responder T-cell functions and the suppressive functions of Treg cells (PMID: 16868552).