Involved in lymphocyte proliferation and functions as a signal transmitting receptor in lymphocytes, natural killer (NK) cells, and platelets. Homodimer; disulfide-linked. Expressed on the surface of activated T-cells, B-cells, natural killer cells, neutrophils and platelets. Earliest inducible cell surface glycoprotein acquired during lymphoid activation. By the activation of T-lymphocytes. Constitutive Ser/Thr phosphorylation in both mature thymocytes and activated T-lymphocytes. CD69