Siglec-15 is a transmembrane glycoprotein belonging to the Siglec family, recognized for its sialic acid-binding properties and role in immune regulation. Structurally, its extracellular domain contains two Ig-like domains, and it functionally associates with the signaling adaptors DAP12 and DAP10. Siglec-15 is expressed on immune cells, including osteoclasts, macrophages, and dendritic cells. It exerts its function by binding to its ligand, the sialyl-Tn antigen. This interaction is critical for inducing TGF-beta production by tumor-associated macrophages and plays an essential role in osteoclast formation and TRANCE/RANK Ligand signaling.