GITR (glucocorticoid-induced tumor necrosis factor receptor), also known as AITR and TNFRSF18, is a 40 kDa transmembrane glycoprotein that functions in immune regulation (1, 2). Mature human GITR consists of a 137 amino acid (aa) extracellular domain (ECD) with three tandem TNF R cysteine-rich repeats, a 21 aa transmembrane segment, and a 58 aa cytoplasmic domain (3, 4). Within the ECD region, cyno GITR shares 89.8% and 53.2% aa sequence identity with human and mouse GITR, respectively. Alternative splicing generates an isoform with a short deletion in the cytoplasmic domain and a potentially secreted isoform that is substituted within the third TNF R repeat and lacks the transmembrane and cytoplasmic regions. GITR is expressed on CD4 ⁺CD25 ⁺ regulatory T cells (Treg) as well as on subsets of thymocytes, lymph node cells, and splenocytes (4-6), and it is up-regulated on antigen-activated conventional CD4 ⁺ and CD8 ⁺ T cells (3, 4, 6, 7). GITR binding by GITR Ligand/TNFSF18 co-stimulates the proliferation and activation of CD4 ⁺ or CD8 ⁺ conventional T cells (3, 7-9). It also induces the proliferation of Treg (8, 10) but inhibits the ability of Treg to suppress immune responses (5, 8, 11-13). This can result in the development of autoimmunity, increased tumor cell killing by effector T cells (5, 11), and increased inflammation in arthritis, allergic asthma, and inflammatory bowel disease (10, 14). GITR is also expressed on sympathetic neurons where it enhances NGF-induced neurite outgrowth and branching (15).