CD134 is a type I integral membrane protein. This receptor is expressed on activated CD4+and CD8+T cells and B cells. The CD134 binds to CD134 ligand (CD252) to provide a costimulatory signal that is independent of CD28. CD134 is involved in coordinating CD4 T cell selection, migration and cytokine differentiation in T helper (Th)1 and Th2 cells. CD134 is also involved in the stimulation of T cells, T dependent humoral response and generation of optimal CD4+ T cell responses in vivo and in vitro. CD134 is expressed on activated CD4+ T lymphocytes, and its ligand, CD134L, is found preferentially on activated B cells. Engagement of CD134 with its ligand, CD134L, delivers a strong costimulatory signal to effector T cells.