T cell immunoglobulin and mucin domain 3 is a member of the TIM family of immune regulating molecules. Mature cynomolgus TIM3 consists of a 182 amino acid (aa)extracellular domain (ECD), a 21 aa transmembrane segment, and a 78 aa cytoplasmic tail. TIM3 is up-regulated on several populations of activated myeloid cells (macrophage, monocyte, dendritic cell, microglia, mast cell) and T cells (Th1, CD8+, NK, Treg). Its binding to Galectin9 induces a range of immunosuppressive functions which enhance immune tolerance and inhibit anti-tumor immunity. TIM3 ligation attenuates CD8+ and Th1 cell responses and promotes the activity of Treg and myeloid derived suppressor cells. TIM3 interactions with Galectin-9 can trigger immune stimulatory effects, such as the coactivation of NK cell cytotoxicity.