OX40 Ligand (OX40L), also known as gp34, is a member of the TNF superfamily. Human OX40L shares 46% amino acid sequence identity with its mouse counterpart. OX40L is expressed on the surface of activated B cells, T cells, dendritic cells and endothelial cells. Similar to other TNF superfamily members, membrane-bound OX40 Ligand exists as a homotrimer. OX40L binds to OX40 (CD134), a member of the TNF receptor superfamily, which is expressed predominantly on activated CD4+ T cells.