Tumor necrosis factor receptor superfamily member 4 (OX40, CD134), a type 1 transmembrane glycoprotein, is predominantly expressed by T cells (regulatory T phenotypes and effector T cells). The OX40 and its ligand OX40L (CD134L, CD252) could be expressed by different cells, including mast cells, endothelial cells, activated natural killer (NK) cells, dendritic cells (DCs), B cells, microglial cells, activated T cells, and Foxp3+ regulatory T cells.