The 25F.1D6 monoclonal antibody specifically recognizes CD366 which is also known as TIM-3 (T-cell immunoglobulin and mucin domain-containing 3, T-cell immunoglobulin mucin receptor 3, or T-cell membrane protein 3). CD366 (TIM-3) is an ~31 kDa type I transmembrane glycoprotein encoded by Havcr2 (Hepatitis A virus cellular receptor 2) that belongs to the TIM family within the Ig superfamily. CD366 (TIM-3) is comprised of one IgV-type domain followed by a serine/threonine-rich mucin stalk region in its extracellular region, a transmembrane segment, and a tyrosine phosphorylation motif in its cytoplasmic tail. CD366 (TIM-3) expression is upregulated on subpopulations of activated myeloid cells including macrophages, monocytes, dendritic cells (DC), microglia, mast cells as well as on Type-1 CD4+ (Th1-like) T cells, cytotoxic CD8+ T cells, regulatory T cells (Treg), and natural killer (NK) cells. CD366 (TIM-3) functions as an inhibitory receptor that helps maintain immunological homeostasis and self-tolerance. It may also serve an immune checkpoint molecule that inhibits antitumor immunity and promotes T cell exhaustion. Crosslinking of cell surface CD366 (TIM-3) by Galectin-9 binding downregulates Th1-like and CD8+ T cell responses and can promote Treg or myeloid-derived suppressor cells. CD366 (TIM-3) enables DC to bind phosphatidyl serine expressed by apoptotic cells, phagocytize these cells to suppress inflammation and promote antigen cross-presentation. CD366 (TIM-3) can also bind to high mobility group protein B1 (HMGB1) and inhibit stimulation of the immune response to nucleic acids released by dying tumor cells.