CD252 is a 35 kD member of the TNF superfamily also known as OX40L and CD134L. The OX40 ligand is expressed on activated B cells and antigen presenting cells. OX40 ligand interacts with OX40 antigen (CD134) expressed predominantly on activated T cells to increase proliferation and IL-2 production, and to enhance proliferation and immunoglobulin secretion in activated B cells. The RM134L antibody can inhibit OX40/OX40 ligand interactions and block the costimulatory activity of OX40L.