The RMT3-23 monoclonal antibody reacts with mouse TIM-3 (T cell immunoglobulin and mucin domain-3) also known as CD366. TIM-3 is a 60 kDa member of the TIM family of immune checkpoint receptors and exists as a type I transmembrane glycoprotein with a mucin-like domain in its extracellular portion and a tyrosine phosphorylation motif in its cytoplasmic portion. TIM-3 is specifically expressed at high levels on the surface of Th1 lymphocytes whereas Th2 lymphocytes express TIM-1 and TIM-2. TIM-3 activation occurs via binding to the cell-associated C-type lectin galectin-9. Upon binding TIM-3 induces apoptosis of Th1 cells. Inhibition of TIM-3 signaling in mice has been shown to exacerbate experimental autoimmune encephalomyelitis, promote IFNγ production and Th1 cell proliferation. Tim-3 has also been shown to be required for the induction of tolerance, as both TIM-3 knockout animals and mice treated with TIM-3-Ig fusion protein display defects in the induction of antigen-specific tolerance. Additionally, TIM-3 signaling is currently being explored as a cancer immunotherapy target as CD8 T cells which express both TIM-3 and PD-1 exhibit greater defects in both cell-cycle progression and effector cytokine production than cells that express PD-1 alone. The RMT3-23 antibody acts as a TIM-3 receptor antagonist and has been shown to have functional activity including suppressing tumor cell growth in a murine sarcoma model.