CD165 is a 37-42 kD transmembrane glycoprotein, also known as AD2, gp37, and SN2. It is expressed on immature thymocytes, most platelets, thymic epithelial cells, fibroblasts, and acute T-lymphoblastic leukemia (T-ALL) cells. CD165 has also been found to be expressed on monocytes and circulating T cells. In addition, CD165 has been detected on islet cells of the pancreas, Bowman's capsule of the kidney and on central nervous system neurons. SN2 antigen has been reported to be identical, or very similar, to the T-cell acute lymphoblastic leukemia antigen, TALLA-1. The biological function of CD165 is still not clear, though it has been proposed that CD165 plays a role in cell adhesion between thymocytes and thymic epithelial cells during development.