Involved in the costimulatory signal essential for T lymphocytes activation. T-cell proliferation and cytokine production is induced by the binding of CD28 or CTLA-4 to this receptor. Expressed on activated B-cells, gamma interferon stimulated monocytes and non-circulating B-cell malignancies. Expressed between 4 and 12 hours post-activation. Protein was detected at cell surface at 24 hours and it's expression was maximal from 48 to 72 hours post-activation.