The 5D12 monoclonal antibody specifically recognizes CD366 which is also known as TIM-3 (T-cell immunoglobulin and mucin domain-containing 3) or T-cell membrane protein 3. TIM-3 is encoded by Havcr2 (hepatitis A virus cellular receptor 2). TIM-3 is a type I transmembrane glycoprotein that belongs to the human TIM family within the immunoglobulin superfamily, having one Ig-like V-type domain in its extracellular region. TIM-3 is expressed on activated monocytes, macrophages, dendritic cells, microglia, and mast cells. It is also expressed on type-1 CD4+ (Th1-like) T cells, CD8+ T cytotoxic cells, regulatory T cells (Treg), and natural killer (NK) cells. TIM-3 functions as an inhibitory receptor which helps cells maintain immunological homeostasis and self tolerance. Crosslinking of cell surface TIM-3 by Galectin-9 binding downregulates Th1-like and CD8+ T cell responses and can promote Treg or myeloid-derived suppressor cells. TIM-3 enables dendritic cells to bind phosphatidyl serine expressed by apoptotic cells and to phagocytize these cells to quell potential inflammation. TIM-3 also binds to the alarmin HMGB1 thereby preventing Toll-like receptor (TLR) by released tumor cell DNA.