This gene encodes a highly sialylated glycoprotein that functions in antigen-specific activation of T cells; and is found on the surface of thymocytes; T lymphocytes; monocytes; granulocytes; and some B lymphocytes. It contains a mucin-like extracellular domain; a transmembrane region and a carboxy-terminal intracellular region. The extracellular domain has a high proportion of serine and threonine residues; allowing extensive O-glycosylation; and has one potential N-glycosylation site; while the carboxy-terminal region has potential phosphorylation sites that may mediate transduction of activation signals. Different glycoforms of this protein have been described. In stimulated immune cells; proteolytic cleavage of the extracellular domain occurs in some cell types; releasing a soluble extracellular fragment. Defects in expression of this gene are associated with Wiskott-Aldrich syndrome. [provided by RefSeq; Sep 2017]