The monoclonal antibody 4G4 recognizes both membrane bound and soluble human CD73, also known as ecto-5’-nucleotidase. CD73 is a 70-kDa GPI-anchored cell surface molecule and belongs to the 5’-nucleosidase family. CD73 is useful as marker for lymphocyte differentiation. It is abundantly expressed on the vascular endothelium and at a low level on certain subpopulations of human lymphocytes. Like many glycosyl-phosphatidylinositol (GPI)-anchored molecules, it transmits potent activation signals in T cells when ligated by antibodies. CD73 hydrolyzes extracellular nucleotides into membrane permeable nucleosides. Ecto-5’-nucleotidase activity is an important mediator of the anti-inflammatory effect by converting extracellular AMP into a potent anti-inflammatory substance adenosine. CD73 has been shown to function as a co-stimulatory molecule in human T cells and to have a role in regulating lymphocyte adhesion. Triggering of CD73 on the surface of lymphocytes, but not on endothelial cells, results in the shedding of the CD73 and increased adhesion of lymphocytes to endothelium via LFA-1 clustering. Furthermore, CD73 has been implicated to mediate homing of skin-infiltrating lymphocytes in vivo.In B-cell chronic lymphocytic leukemia the expression of CD73 is decreased. Besides this, CD73 activity has been implicated as sensitive and useful indicator for mild zinc deficiency.The monoclonal antibody 4G4 causes a reduction in CD73 expression on lymphocytes, reduces enzyme activity, and inhibits the binding of lymphocytes to endothelial cells.