The protein encoded by this gene is a major sialoglycoprotein found on the surface of thymocytes, T lymphocytes, monocytes, granulocytes, and some B lymphocytes. It may be part of a physiologic ligand-receptor complex involved in T-cell activation. During T-cell activation, this protein is actively removed from the T-cell-APC (antigen-presenting cell) contact site, suggesting a negative regulatory role in adaptive immune response. [provided by RefSeq, Sep 2011].