– Siglec-15 is a transmembrane glycoprotein in the Siglec family of sialic acid-binding immune regulatory molecules. Mature human Siglec-15 consists of a 244 amino acid extracellular domain (ECD) with two Ig-like domains, a 21 aa transmembrane segment, and a 44 aa cytoplasmic domain. Siglec-15 is a potential therapeutic target for osteoporosis and plays a conserved regulatory role in the immune system of vertebrates..