CD134, also known as OX40 or tumor necrosis factor receptor superfamily member 4 (TNFRSF4), is a type 1 transmembrane glycoprotein and a secondary co-stimulatory immune checkpoint molecule involved in long-term T cell immunity. It is transiently expressed on activated CD4+ and CD8+ T cells after T cell receptor (TCR) ligation. The interaction between CD134 and its ligand, OX40L, enhances T cell activation, proliferation, survival, and cytokine production. CD134 is crucial for maintaining an immune response beyond the initial activation phase and is being investigated as a target in cancer immunotherapy, autoimmune disease modulation, and vaccine development.