OX40 (CD134), a member of the tumor necrosis factor (TNF) family, is mainly expressed on activated T-cells (including CD4, CD8 T-cells), Th2, Th1, Th17, Treg cells, NK cells, NKT cells and neutrophils. OX40 interacts with OX40 ligand, which is typically expressed on APCs. The interaction between OX40 and its ligand induces the expansion and proliferation of T-cells whereas it decreases the immunosuppression of Treg cells. As such, it enhances the immune response to a specific antigen. Given the fact that CD40 is a potent immune-stimulating target, it may play an important role in anti-tumor therapy.