The clone C-813.21, a mouse monoclonal antibody selectively recognizes a 110 kDa type 1 transmembrane glycoprotein composed of disulfide-linked 50-55 kDa monomers surface antigen known as CD27. C-813.21 belongs to the TNF-receptor superfamily and is widely expressed on all mature T cells, a subset B cells, and NK cells. CD27 plays an important role in the co-stimulation of T cell activation and regulation of B cell differentiation and proliferation.