CD134, also known as OX40 or TNFRSF4, is a co-stimulatory molecule belonging to the tumor necrosis factor receptor superfamily. It is primarily expressed on activated CD4+ and CD8+ T cells, as well as other immune cells like natural killer (NK) cells and dendritic cells. CD134 plays a crucial role in enhancing T-cell proliferation, cytokine production, and survival by interacting with its ligand, CD134L (OX40L), which is mainly expressed on antigen-presenting cells (APCs) such as dendritic cells and macrophages. This interaction promotes T-cell-mediated immune responses and helps maintain memory T cells, making CD134 an important target for immunotherapy. In cancer treatment, agonistic antibodies targeting CD134 have been shown to enhance anti-tumor immunity by boosting T-cell activity and overcoming immune suppression.