CD278, also known as ICOS, is a 50–60 kDa homodimeric membrane glycoprotein and a member of the CD28 family reacting with the inducible co-stimulatory (ICOS) molecule. It is highly expressed on activated T cells. It is the receptor for B7-related protein 1 (B7RP-1). Like CD28, ICOS is a co-stimulatory signal for T cell activation and proliferation and cytokine production. It is not expressed on resting or activated B cells, monocytes, NK cells, granulocytes, dendritic cells or platelets. Unlike the constitutively expressed CD28, ICOS expression is de novo. It has been suggested that ICOS may play an important role in IL-10 production. In the presence of IL-10, purified recombinant human ICOS significantly increased in vitro B cell growth stimulated by pokeweed mitogen (PWM) and enhanced production of IgG. Clone D1K2T recognizes endogenous levels of total ICOS protein.